The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music - Gene Wallenstein (2008)

Part III. The Pleasure Instinct and the Modern Experience

Chapter 9. Pleasure from Proportion and Symmetry

Our inner faculties are adapted in advance to the features of the world in which we dwell. . . . Our various ways of feeling and thinking have grown to be what they are because of their utility in shaping our reactions on the outer world.

—William James

There must be in our very nature a very radical and widespread tendency to observe beauty, and to value it. No account of the principles of the mind can be at all adequate that passes over so conspicuous a faculty.

—George Santayana

As a boy, the young Charles Darwin showed no signs of the brilliance that was to shine by his twenty-fifth birthday. He and his older brother, Erasmus, grew up playing along the banks of the Severn River in Shropshire, the idyllic countryside setting of Jane Austen’s Pride and Prejudice. By most accounts he was an amiable if not astute child who could just as likely be found digging for beetles as attending to the lessons foisted on him by his Latin tutors.

His father, Robert, was a physician and expected the same from both of his sons, who were shipped off to Edinburgh to study medicine in 1825. At the tender age of sixteen, the youthful Charles Darwin was quite shocked by the pace of the city, exposed to a side of life for which he had little conception. The university at the time was the center of a raging debate over Scottish nationalism that seemed an unending battle for both God and country. The greens and lecture halls were filled with rabble-rousers, each coddling their own theological baby, Jacobites, Calvinists, Loyalists, all willing to argue their case. Nor were the faculty, lecturers, and readers immune to this carnival of ideas, some of which proved very dangerous indeed.

Charles learned that his grandfather Erasmus, who died several years before his birth, had somewhat of a cult following at the university. Erasmus was considered an irreverent man by friends and family, and in his later years lived an unconventional lifestyle, championing a free-love movement of sorts. He was trained as a medical doctor and botanist, and his scientific views on nature and religion were even more scandalous than his personal life. In one of his many poems, titled “The Temple of Nature,” we find the early seeds of a theory of evolution that his grandson would harvest some sixty years later:

Organic Life beneath the shoreless waves 
Was born and nurs’d in Ocean’s pearly caves; 
First forms minute, unseen by spheric glass, 
Move on the mud, or pierce the watery mass; 
Then as successive generations bloom, 
New powers acquire and larger limbs assume.

Darwin returned home following his second year at Edinburgh, and after fretting for days, gathered enough courage to tell the family he was quitting medical school. He was less at ease with medicine after attending surgical grand rounds and witnessing operations that were then conducted without the benefit of local anesthesia—sights that must have been truly horrific experiences. Robert was outraged by what he took as his son’s apathy: “You care for nothing but shooting, dogs, and rat-catching and you will be a disgrace, to yourself and all your family.” He decided that if Charles was not to be a doctor, the only respectable alternative was for him to join the clergy. So the young Darwin set off to study theology at Cambridge University, with the aim of understanding the true design of God’s nature.

It was in nature that Darwin sought God. He spent much of his free time searching for insects in the forests and fields around the university and reading accounts of archaeological expeditions in South America. He was fascinated by the way other cultures lived and the strange places they called home. An adventurous itch began to build in Charles, and when he was given the opportunity to join Captain Robert FitzRoy and the crew of the Beagle on a five-year expedition to map out new trade routes of Argentina and neighboring countries that had just been delivered from Spain’s control, he jumped right in.The twenty-two-year-old boarded the Beagle on December 7, 1831, a beautiful autumn day in Plymouth, prophetically carrying with him for the voyage a copy of his favorite verse, Paradise Lost.

For five years the crew of the Beagle charted out the southern waters, past the Canary and Cape Verde islands, on toward Montevideo, Tierra del Fuego, and around Cape Horn. They veered north through the Strait of Magellan, carefully avoiding the iceberg fields that emanate from the Antarctic Circle to within a few hundred miles of the South American coast in spring. Onward, they skirted the shoreline of Chile, stopping for approximately one month at a tiny group of islands known as the Galapagos, just south of the equator.

The time Darwin spent on these islands gathering specimens and the details that he recorded of how functionally well adapted each species seemed to be to its environment had a profound impact on his thinking about evolution, but not until some two years after the visit. While on the islands he recorded a rich variety of birds, particularly the finches, wrens, and warblers that, surprisingly, looked almost identical except for their differing beaks. Some had large, blunt beaks that were used for crushing large seeds, while others had rather narrow and elongated beaks (think needle-nose pliers) that were used to extract small seeds and insects from difficult-to-reach places. Darwin dutifully noted these distinctions and sailed on, returning to England in the fall of 1836 a changed man.

Once home Charles began to receive reports from the leading British zoologists about the specimens he brought back from his travels. These caused confusion at first, but a theory soon emerged that would change the world, and it stemmed from the differences in the birds he saw in the Galapagos.What Darwin failed to realize at the time, but soon learned after returning to England, was that these were all finches. The primary factor distinguishing them was beak morphology. It was puzzling how so many different species of finch came to inhabit such a small area, and he reasoned that perhaps they all descended from a common ancestor and gradually, after many generations, began to diverge in appearance. He thought of Lamarck and his “transmutation of acquired characteristics,” but he did not accept the idea that changes accumulating within a lifetime could be passed down to offspring. Alternatively, he speculated that the different beak shapes must give each finch a special advantage to living in its local environment. For example, finches with long, narrow beaks would have an advantage in securing food in places that might not be reached by a finch equipped with a larger beak. With a more reliable food supply, the long-beaked finches would have an edge over their natural competitors and be more likely to reproduce. Likewise, in other parts of the islands where elderberry brush was plentiful, finches with more powerful and compact beaks dominate, since they alone can manage to grind the hard casing of their seeds to an edible pulp.

Darwin also knew what farmers understood for years—offspring tend to resemble their parents. Farmers select crops for their next harvest by determining which strains produced the best product this year and replant them hoping to build on their success in future seasons. Eventually this process of farmer selection results in enough accumulated differences that entirely new varieties emerge. In an instant of recognition, Darwin was terrified by the implications of his theory. If he was right, finches (and other animals) are selected by natural competition for food, sex, water, and all means of subsistence, and those that just happen to have an adaptive edge (perhaps because of a longer beak, or keener eyesight, or faster flight) are more likely to reproduce and bear similar offspring, thus ensuring a continuation of that lineage. Other, less successful finches that do not possess the advantages are less likely to survive to reproductive age in that environment, thus minimizing the spread of their characteristics to offspring. The different varieties of finches he saw on the islands were functional success stories—the good seeds. Each had some adaptations that secured reproductive success in their given environment at the expense of competing birds.

This was a deeply sad story for Charles, a religious man who had great difficulty accepting the idea that finches and other birds—indeed, all animals including humans—must evolve over time. Animal forms are not fixed through eternity, but rather molded by a process of natural selection—imagine, selection of a species without a Selector. It took Darwin fourteen years of confidence-building before he made his theory public in the magnum opus On the Origin of Species by Means of Natural Selection. The public outcry of blasphemy was, as expected, enormous, and included many of his scientific colleagues, friends, and even family (his wife wasn’t thrilled with the theory). Of course, an unwillingness to accept species selection without divine intervention continues today.

Darwin concentrated on evolution by natural selection in Origin of Species, which made virtually no reference to human behavior. This is not an oversight. Darwin was troubled by the lack of an obvious way to account for the development of so many uniquely human activities, such as making music and art, by the theory of natural selection. What is the survival function of singing a pretty melody, making someone laugh, being able to tell a good story, or creating a work of art? None of these human qualities seemed to fit into the same theory as that of the finches growing different-size beaks to adapt to distinct environmental challenges.

In his follow-up work The Descent of Man and Selection in Relation to Sex, Darwin reconciles the appearance of these behaviors by developing the theory of evolution by sexual selection—a special form of natural selection. The bottom line of evolution is the survival of genes down through the generations.An organism may survive to a ripe old age, but if it fails to reproduce, its genes die right along with it. For selection to drive evolution, an organism must survive and reproduce.

The adaptations that result from sexual selection are referred to as ornaments by evolutionary biologists. The example given earlier was of the peacock’s enormous and well-decorated plumage, which plays a direct role in attracting the attention of peahens. Darwin argued that the evolution of this display was driven by female choice.

In species where there is competition for mate selection, the elaboration of secondary sexual characteristics (that is, those not serving a direct function in reproduction) usually occur in males to vie for the attention of females, who have a greater metabolic investment in reproduction.

The English geneticist Angus Bateman observed that in many species, females bear a much larger burden for producing an offspring than males. This inequity begins with the production of sex cells. Women produce approximately four hundred nutrient-rich ova during an entire lifetime, while men produce billions of sperm that are replenished at a rate of about eleven million to twelve million per hour. A female has a relatively small number of eggs at any given time, and a single male can fertilize all of them; hence the female will not produce more offspring by mating with more than one male. Contrasting this, males are capable of fathering many more offspring than any one female can bear if he mates with several at a time. Of course, fertilization and gestation occur internally within females, consuming great amounts of time and metabolic resources. If a successful birth occurs, this is followed by potentially several years of lactation to feed the child and still many additional years of investment in raising the toddler to an autonomous age. Thus, in many species where these conditions exist, females must be far more choosy in selecting mates than males.

Female choice has been shown to drive a broad range of adaptive traits or ornaments in males. A key question that has troubled biologists for decades is how particular traits are selected in the first place. For a trait to form through sexual selection there must be some initial preference for it by the female. Early theorists such as Sir Ronald Fisher, a geneticist who also made pioneering contributions to modern statistics, argued that the initial preference might be completely arbitrary. Say, for example, that a small number of female peahens developed a preference for mating with peacocks with brighter-than-usual plumage. Peacocks having brighter plumage would be more likely to mate with these peahens and produce peacock offspring with brighter plumes and peahens with a preference for bright plumes. Assuming the traits (producing the bright plume and preferring the bright plume) become genetically correlated, this would produce a positive feedback system where both traits become increasingly magnified over many generations until plumes become so large that they begin to have survival costs associated with their production that temper the process. At this point, greater elaboration of the plume should be curtailed, since it will result in survival deficits. An equilibrium point would emerge where the positive sexual selection effects of large, bright plumes are perfectly balanced by their negative survival costs.

But why would peahens develop a preference for larger, brighter plumes in the first place? We can find some help here in the work of biologists such as William D. Hamilton and Marlena Zuk, who pointed out that many ornaments are excellent indicators of genetic fitness. The fitness indicator theory suggests that any population where there is an imbalance of investment in producing offspring should theoretically result in the sex with the greater investment also having more at stake in mate selection. This would create pressure for that sex to be able to identify and pair with the most genetically fit mate available. In this example, female choice involves finding the best male genes with which to join with her own so that her offspring have the best chance of survival and reproducing. Of course, finding the best genes is tricky.

One argument might be that the best male genes are those that are fairly different from the female’s own, since this will reduce the possibility of recessive combinations being expressed (genetic diversity prevailing over homogeneity).Another argument might be to simply find genes that are healthy in general. Either way, it has been shown that in many sexually reproducing animals (including humans), key traits that are reliably associated with genetic fitness can be detected and used for mate selection.

The fitness indicator theory of sexual selection is fairly convincing, since it explains a great deal of empirical observations across many species where the development of a particular trait seems to have extended well beyond the limits imposed by survival costs. The case of the peacock’s plume is a perfect example in this regard, since its exaggerated growth makes it a target for predators. Indeed, the handicap principle championed by biologist Amotz Zahavi (see chapter 8) says that it is exactly this cost that makes it a relevant fitness indicator.

Fitness indicator theory goes a long way in explaining why some traits seem to be taken to extremes beyond which survival costs would be accrued. It also helps explain how initial female preferences for a trait, if reliably associated with genetic fitness, can emerge. Peahens that have preferences for traits that have poor or no correlation with genetic fitness are in trouble. If they use these traits for mate selection, they are essentially gambling with their reproductive success. If they have chosen poorly, their genes, along with the trait preferences they support, will be less likely to thrive and be propagated.

In this context it is easy to imagine how pleasure might play a pivotal role in this process. As we have seen in the chapters to this point, the pleasure instinct drives the emergence of distinct receiver biases—preferences for certain forms of sensory stimulation that are critical for normal brain development and maturation. If pleasure-associated preferences for particular forms of stimulation guide an organism toward traits that are also good fitness indicators, this combination may prove to be very useful during mate selection.

Imagine Sally is being pursued by Harry. If Sally makes her selection of a mate purely on the basis of fitness indicators without regard to whether they bring her pleasure, she might do so by simply summing up the tally and attributing an equal value to each indicator. A completely different approach would be to use the pleasure associated with the appearance of a particular fitness indicator to gauge its current importance relative to others. In this context, pleasure is the common currency for organizing and prioritizing competing goals and interests. Using this approach, Sally would be able to assess and rank-order which fitness indicators are more important than others so she could make a more informed choice that matches her current needs. This process would be far more flexible and adaptive to changing environmental circumstances than one based on a simple summation of overall values across all indicators.

If, at this particular time, Sally has not eaten for two days and is faced with a choice between Harry, who has taken her to dinner, and Tom, who has taken her to a movie sans dinner, the choice will be very different depending on the approach. If she chooses solely on the basis of fitness indicators, either Harry or Tom will do, since they have both done things to display their fitness. Harry has shown that he can provide food, a critical resource for survival. Tom has shown that he has sufficient wealth that he can waste money on things not directly related to survival. Both have provided evidence of their fitness. If forced to choose simply on the basis of fitness indicators, Sally might have to toss a coin to decide. A different outcome would occur, however, if pleasure is used to rank-order the relative importance of fitness indicators to match her current needs. In this scenario, Harry is the clear winner, since he has delivered what she needs most at present—nutrition.

Seen in this light, pleasure provides the common reinforcement mechanism to drive and align motivated behaviors that may require very different forms of learning and that likely occur in different sensory systems. Pleasure is nature’s shortcut; it enables humans to respond quickly to changing life demands by prioritizing basic needs that involve different neural systems on a single metric. The brain’s motivational systems for signaling hunger for food and hunger for sex are distinctly different. Both brain systems, however, interact with the brain-stem pleasure circuitry we have been discussing. The pleasure system, in this respect, is the common denominator that allows a direct comparison of the needs associated with both competing hungers so that appropriate behavior can be chosen based on current priorities.

What are some examples of receiver biases (see chapter 8) crafted by the pleasure instinct that are also good fitness indicators? Certainly not all fitness indicators are consistent with the receiver biases created by the pleasure instinct. Likewise, most of the receiver biases we have discussed thus far are good fitness indicators, although there are, of course, exceptions. Let’s look at a few classic archetypes to guide our thinking.We will see that such biases occur in both sexes in humans, since mating generally involves monogamous pair bonding.

The Hidden Persuaders

It is clearly not a controversial claim to suggest that different cultures and different times tend to produce variable ideas of what is most physically attractive in a potential mate. Civilizations from the ancient Greeks onward have attempted to formulate canons for defining the ideal of physical beauty. Plato and Plotinus wrote extensively about the geometry of physical form and emphasized the inherent aesthetic appeal of things that exhibit strong symmetry, harmonious proportion, and vivid color. The emphasis on symmetry and proportion—elements that are quantifiable—began in the fifth century B.C. and has been built upon steadily by artists and philosophers ever since.

During the Renaissance, an explosion of aesthetic theories applied to human forms emerged, many of which concentrated on identifying the proper metric to measure true beauty. Leonardo da Vinci and Albrecht Dürer were among the most popular artists who proposed geometric systems for measuring beauty based on symmetry and proportionality of body parts. During this period, a large number of measurement systems were proposed, each one emphasizing a particular set of metrics. For instance, Dürer, inspired by the great Italian artist Jocopo di Barbari, created a formal theory of beauty based on anatomical proportions such as finding correspondence between finger length and palm width, arm length to an even-integer ratio of body length, and so forth. Although some artists from this period, such as Leon Battista Alberti, believed there was a single irreducible geometric form representing perfect beauty, da Vinci and Dürer were more accepting of relative beauty in that many forms could be seen as being equally beautiful provided a few basic ratios were preserved.

The canonization of beauty continues today in modern attempts to determine if a universal definition can be formulated that is consistent across cultures. While it has proven difficult to show that any of the historical canons match up with what modern people actually find attractive, there have been interesting findings from some of these studies.

What we generally find in modern studies is that there is no ideal physical form that all will agree on as being beautiful based on pure mathematical principles—no golden ratio of beauty. However, there do seem to be certain physical traits that people from widely diverse cultures (Western, Middle Eastern, Eastern, north and south of the equator) agree on as being beautiful or attractive in a potential mate.The questions are:What are they? Why do we like these traits so much?

Pleasure from Proportion

Let’s start with generic body form. The most obvious quantifiable traits that characterize body appearance are weight and height. When one looks just within a single culture, it is easy to find that certain height and weight ranges are thought to be more attractive than others. In parts of North America and Europe, there tends to be a preference for tall and thin models (of both sexes), while in some South American and Polynesian cultures, those with a little more weight are considered most attractive. Given this variability for ideal height and weight by different cultures, these are clearly not universally accepted traits for beauty. A young woman raised in the Bronx might look at a possible suitor who is tall, dark, and handsome with a winsome eye, yet the same man might be seen as meek and too skinny for the likes of a highlander from Papua New Guinea. Indeed, the man might be seen as meek and too skinny for the likes of another Bronx native. Barring extremes, it turns out that body weight and height are fairly poor predictors of whether an individual is generally found to be attractive.

What seems to matter most in determining attractiveness is the overall body shape of a person. People of the same height and weight can have bodies that look remarkably different. Body shape is driven by the distribution of body fat, and as we will see, this trait is significantly correlated with a woman’s sex hormone profile, reproductive capability, and risk of disease. In humans, the distribution of body fat depends on both age and gender. Boys and girls have strikingly similar distributions in infancy and early childhood.At puberty, hormonal changes lead to a shifting of these distributions. Increased estrogen in postpubertal girls blocks fat buildup in the abdomen and stimulates buildup in the buttocks and thighs. Increased testosterone in postpubertal boys does quite the opposite, causing increased fat deposition in the abdomen and decreased buildup in the buttocks and thighs. In general, women have greater amounts of fat in the lower parts of the body (gynoid or pear shape), and men have greater amounts in the upper portions of the body (android or apple shape).

Differences in body shape can be reliably quantified by measuring the circumference of the waist and hips, and calculating a waist-to-hip ratio (WHR). Male and female prepubertal children have similar WHRs. After puberty, gender-specific hormonal changes shift fat distributions within each sex such that a woman’s WHR tends to be smaller than that of a man. Healthy premenopausal women typically have a WHR from .67 to .80, whereas healthy men usually have WHRs between .85 and .95. Hence, in women, WHR can be used as a reliable means to gauge an individual’s general reproductive status (pre- versus postpubertal).

In the early 1990s, psychologist Devendra Singh of the University of Texas began publishing a series of papers demonstrating that men and women from very different cultures display remarkable similarity in what WHR they find attractive in a female. You may find it odd that Singh used both males and females in his studies of female attractiveness, but this is a critical control from an evolutionary perspective. If a trait is to be a reliable marker of attractiveness, both the signal receivers and the signal generators must be aware of its meaning. Woman, realizing that a particular trait is seen as attractive by men, might wish to accentuate or attenuate it by various means (for example, makeup, clothes, posture, and so forth) based on the desire to indicate sexual availability.

In his first series of studies, Devendra Singh created a set of line drawings depicting women with three body weight categories (underweight, average, and overweight). Within each weight category, he used line drawings to represent four WHRs. Two were typical gynoid shape with WHRs of .7 and .8, and the other two had a typical android shape with WHRs of .9 and 1.0. He and his colleagues showed the drawings to men and women of different ages (eighteen to eighty-five years old), professions, educations, and ethnicities, and asked them to rate each figure based on its attractiveness. The results were very interesting. Men and women rated drawings with a WHR of .7 as the most attractive within each weight category. The drawing seen as most attractive was of a female figure with a WHR of .7 and average weight.The drawings seen as least attractive were of female figures with .9 and 1.0 WHRs from the overweight category.

A possible critique of these findings might be that they have focused entirely on Western cultures, since Singh’s studies only included Americans of European, Mexican, and African descent.To show that a preference for a particular trait is shaped by a selection process, the first step is to demonstrate that it is at work in humans of diverse cultures and ethnic groups. To see if WHR is indeed a universal marker of attractiveness, Singh next did a series of studies using men and women from nineteen different cultures. In these studies, he included people from America, Europe, Australia, Africa (Kenya, Uganda, Guinea-Bissau), the Azore Islands, the Shiwiar tribe of East Ecuador, Indonesia, China, India (Sugali and Yanadi tribes) Chile, and Jamaica. He showed the same line drawings as used in the earlier studies and asked subjects to rate their attractiveness, healthiness, youthfulness, and desirability as a marriage partner.

Despite coming from incredibly diverse cultural backgrounds, there was a clear preference for a WHR of .7 in each weight class. Moreover, there was a strong correlation among the variables in the findings. Perceived attractiveness, youthfulness, and healthiness were ranked in an almost identical fashion across the different cultures. For these variables, positive rankings decreased systematically with increased WHR.

One might argue that there could still be the influence of Western media on these findings, since it tends to associate a particular body shape with beauty. Granted, but this is unlikely to have impacted these results, since both the Azoreans and subjects from Guinea-Bissau had virtually no exposure to Western media, yet they ranked the drawings similarly to most other cultures, including those from the United States. Hence it seems that WHR may be a universal marker for attractiveness in a potential mate. In females, the maximal ranking seems to be about .7. Of course, as mentioned earlier, many different body types and images of beauty can have a WHR of .7. The classic beauties Marilyn Monroe at 36-24-34 and Audrey Hepburn at 31.5-22-31 had very different hourglass figures, but shared the same WHR of .7.

The next logical question to ask, given the seemingly universal appeal of a .7 WHR in females, is why such a trait is important. Why do we find looking at a female with a .7 WHR more pleasurable than one with a WHR of, say, 1.0? The answer is not to be found in some ancient canon of beauty, prescribed by the divine or mathematical. Rather, it can be found in the way WHR reveals basic information about the bearer’s general health and fecundity. Reproductive success for a man in ancient environments must surely have depended on selecting a mate with good health, genetic fitness, and excellent reproductive capacity. Of course, these characteristics are not directly observable; hence sexual selection has shaped certain mental mechanisms for measuring genetic fitness indirectly. There is now convincing evidence demonstrating that WHR is a fairly good predictor of long-term health risk, mortality, and reproductive endocrinological status.

Women with a WHR lower than .8 have a significantly reduced risk relative to women with a WHR above .8 for key conditions that are known to hinder reproductive success and fertility, including hyperandrogynism, menstrual irregularity, suboptimal sex hormone profiles (optimal is high estrogen and low testosterone), and abnormal endocervical mucus pH. Women with WHRs below .8 are also significantly more likely than their age-matched counterparts with WHR above .8 to have a successful pregnancy outcome after artificial insemination or in vitro fertilized embryonic transfer. Thus WHR is a reliable marker for estimating a woman’s reproductive health.

It is important to note that these are statistical observations. Certainly, there are many women with WHR well above .8 who have perfect reproductive and general health. Likewise, there are undoubtedly loads of women with WHR of .8 and below who do not share this health. Sexual selection shapes traits that are expressed to various degrees within a population. It is nature’s way of playing the odds. If a healthy man mates with a female with a WHR of .7, there is no guarantee of offspring. Sexual selection has crafted a psychological mechanism—a preference for females with a particular WHR range—that, all other things being equal, increases the odds of producing offspring who will preserve his genes, since the female has better odds of increased genetic fitness, disease resistance, and fecundity.

Seduction by Symmetry

Another excellent example of a receiver bias crafted by the pleasure instinct that does double duty as a fitness indicator is our love of symmetry. In his book The Descent of Man and Selection in Relation to Sex, Charles Darwin wrote, “The eye prefers symmetry or figures with some regularity. Patterns of this kind are employed by even the lowest savages as ornaments; and they have been developed through sexual selection for the adornment of some animals.” As we saw in the previous chapter, at about the time the primary visual cortex (V1) reaches maximal cell proliferation, babies begin to become keenly attracted to objects exhibiting strong lateral symmetry. As if on cue, this preference emerges just at the right time to promote experience-expectant synaptic pruning of V1 and downstream visual cortical areas, hence promoting normal maturational development. The pleasure obtained in self-stimulating V1 and downstream visual areas with highly symmetric objects as an infant forms the basis for a host of preferences as adults, including an attraction to things bearing strong lateral, rotational, and radial symmetry. The most obvious example of this is how this preference impacts our choice of mates.

As we have been discussing, the fitness indicator theory of sexual selection suggests that individuals develop preferences for potential mates who possess traits that are reliably linked with good genetic quality and increased likelihood of bearing viable and vigorous offspring. Indeed, many sexually selected traits (such as WHR) seem to have comparable genetic variability to that commonly observed for fitness traits (for example, fecundity), which indicates they may have evolved as signals of overall phenotypic condition.

One important marker of phenotypic condition that has caught the attention of researchers in the past decade is bilateral symmetry of the body. Imagine a line drawn from top to bottom down the middle of your body. If I were to take calipers and carefully measure the relative size of many body parts (such as the width of your feet, ankles, and ears, or the length of your fingers from both sides), I would typically find slight variations in relative size from left to right. The variations are usually small, on the order of about 1 percent of the overall size of the body part being measured. The distribution of these bilateral differences is referred to as fluctuating asymmetry, since departures from perfect symmetry vary randomly along the body axis. Fluctuating asymmetries are very different from directed asymmetry, such as handedness, since the former average out to zero in the general population.

Fluctuating asymmetry can be caused by a number of factors during development. Since the corresponding sides of the same body part are coded by the same genes, fluctuating asymmetries typically emerge from either environmental stressors or genetic perturbations within the genome that reduce developmental stability. Such stressors include things such as parasites, pathogens, pollutants, and other environmental challenges such as extreme temperatures or marginal habitats. Fluctuating asymmetries also increase with genetic perturbations caused by things such as inbreeding, the presence of certain recessive genes, chromosomal abnormalities, and homozygosity. Considering this, it is thought that fluctuating asymmetry is a measure of the extent to which an individual has been able to maintain a normal developmental trajectory by resisting such challenges. A person with a large number of harmful genetic mutations or who is less able to resist pathogens should on average exhibit greater fluctuating asymmetry.

There is now a substantial body of literature showing that fluctuating asymmetry is a reliable indicator of overall phenotypic quality. Increases are associated with decrements in biological fitness in a number of key domains, including reproductive success, growth rate, an ability to resist disease, metabolic efficiency, immunocompetence, and overall survival rate. Insofar as fluctuating asymmetry has been found to be partly heritable and is a reliable marker of phenotypic quality and biological fitness, several authors have suggested that ultimately it is a marker of genetic quality. As such, theoretical models of sexual selection by mate choice and competition would predict that fluctuating asymmetry should exhibit a strong relationship with mating success.That is, individuals with increased symmetry should, in general, enjoy more successful mating than their more asymmetric counterparts.

Indeed, in the majority of species tested, males with the highest degree of bilateral symmetry tend to have the greatest mating success. In a large-scale review of sixty-five studies involving forty-two species across four major taxa, biologists Anders Moller and Randy Thornhill found a number of interesting results that were predicted by theoretical models years earlier.

First, in the vast majority of species tested—from fruit flies to humans—males showed the strongest association between fluctuating symmetry and measures of mating success. Females also exhibited a statistically significant relationship between fluctuating asymmetry and mating success in many species (especially humans), but the relationship was strongest in males across most species, which is exactly what would be predicted from evolutionary models of sexual selection based on female choice. Choosier females would lead to greater sexual selection in males, who would, in turn, develop more pronounced traits (for example, ornamentation) to be assessed by females as part of their selection criteria. Accordingly, a sexually selected trait such as fluctuating asymmetry should bear a stronger relationship to mating success in males who have to compete with each other for female attention in species where female choice dominates.

In contrast, there should be a more pronounced relationship between fluctuating asymmetry and mating success for females in species where male choice dominates. In species where mate choice is more equitable among the sexes, the relationship between fluctuating asymmetry and mating success should exist in both sexes to roughly equal amounts.

A second important finding was that in most species, including humans, the relationship between fluctuating asymmetry and mating success was stronger for body parts involving secondary sexual characteristics (traits that distinguish the two sexes of a species but that are not directly part of the reproductive system) than for other parts. For instance, in the dozen or so studies of humans, investigators have examined symmetry at several stops along the primary axis of the body, including the feet, ankles, hands, fingers, arms, chest, shoulders, ears, face, breasts, and the overall figure. In terms of gauging mating success, researchers have measured things such as the rated attractiveness of a potential mate, the likelihood of accepting a date with that person, the likelihood of engaging in sex with them, and others. In general, the most persistent and robust relationship between fluctuating asymmetry and mating success in humans was found to involve parts of the body that are most meaningful during intimate encounters, such as the face, shoulders, chest, and breasts. Why should this be the case? One might argue, perhaps, that it is exactly features such as the face, chest, and breasts that naturally draw our attention because we find them pleasurable, so we are inclined to tune in to them when deciding about a possible mate. But this is a circular argument in this context. True, we focus on these features because we find viewing them pleasurable—certainly more so than focusing on a potential mate’s ear length or ankle width. But the primary question is:Why is it so much fun to look at these features in the first place? A second, related question is: Why do these features carry more relevant information (in terms of fluctuating asymmetry) than, say, our feet, knees, or elbows when gauging the suitability of a sexual partner?

When considered from the “good genes” perspective, it would seem that symmetry of secondary sexual characteristics such as the face and breasts varies most strongly with mating success because these features are hard to fake (at least in ancestral times) and are honest indicators of true fitness. For instance, the biological complexity and metabolic cost of building a face makes it particularly susceptible to genetic or environmental perturbations during development that would leave a visual record of such events in the form of increased asymmetry. As discussed earlier, preferred features that are genuinely related to fitness should increase in prevalence, assuming that the preference and appearance of the trait are genetically correlated. This increase in the expression of the trait is tempered by metabolic costs associated with its development. Without such constraints, a simple Fisherian model would predict a runaway process of the trait becoming ever exaggerated and everyone having maximal symmetry throughout the population.This obviously does not occur. Body parts that are the most costly to build are the best candidates for being honest fitness indicators, since they have developed despite metabolic costs and the possibility of environmental or genetic perturbations.

Another important reason why symmetry of secondary sexual characteristics might be more important indicators than symmetry of other body parts is that these features change dramatically at puberty, announcing sexual maturity. Take the face, for example. It is often difficult to determine the sex of a baby by just looking at its face if no supporting clues are available such as gender-typical clothing. Toddlers can also have very similar facial appearances across the sexes, but marked differences in facial appearance generally occur by puberty. During adolescence hormonal changes sculpt these differences. Boys’ faces become larger and more angular, especially the lower jaw and brow ridge. Girls’ faces retain a smoother forehead and smaller lower jaw, giving a rounder impression.A smaller nasal bridge relative to boys gives the impression of larger and wider-spaced eyes in girls. Clearly, the developmental growth of a face involves much more than simply scaling up the size of the prepubertal face.

Many body parts undergo a qualitative change at puberty where fat redistributes in a sex-specific manner. By comparison, however, the face undergoes extreme changes with many more opportunities for environmental and/or genetic challenges to the developmental stability needed to achieve perfect symmetry. This sensitivity to challenges that occur during development is what makes facial symmetry a potential selection mechanism for identifying mates, assuming it correlates with actual fitness. So let’s see what is so important about faces that make them the center of attention in the mating game.

A Fit Face

As we saw in earlier chapters, there is something special about faces that makes them a naturally pleasurable stimulus. Even babies that are a mere ten minutes old gaze longest (a proxy for measuring preference in infants) at illustrations with anatomically correct compositions of a face when compared to control illustrations that have all the same components but are ordered in a random manner.They will also visually track a line drawing of a face at this age. Right out of the womb, babies have a preference for faces. Within another day, newborns develop a preference for their mother’s face as opposed to that of other similarly aged women who have recently given birth. By day three, infants can mimic certain facial expressions, such as sticking out a tongue in response to a similar gesture from Mom or Dad. Add a few months and infants develop an ability to discriminate one unfamiliar face from another and detect different emotional expressions, of which they prefer joyful over angry faces.

There might be many different reasons why faces seem naturally interesting and attention-grabbing to humans. The prevailing theory is that an infant’s fascination with faces emerges as an adaptive mechanism to promote parent-child attachment. Being able to recognize and engage the primary caregiver increases the likelihood that an infant will become emotionally bonded with that individual and receive proper nurturance. The need to recognize, engage, and extract information from faces continues, of course, through childhood and into adulthood. Being able to read the minds of others in a social group is also important for survival and reproductive success. Humans can’t read minds, but the next best thing is being able to understand the emotional mind-set of your peers. No other body part even comes close to yielding such rich emotional information about the bearer as is the case with the face.

The “face as a kin recognition device” theory is well supported in the literature and is consistent with a large number of primate studies, including humans.There may, however, be additional reasons why infants (and adults) find faces so pleasurable. Faces are composite objects made up of smaller, complex stimuli—eyes, lips, nose, jaw, brows, skin, and so forth. Each of these elemental objects is itself a potentially rich source of stimulation for growing brains and indeed has physical characteristics that are known to be naturally preferred at or near birth by newborns. For instance, newborns have a preference for stimuli with strong lateral symmetry and can recognize vertically symmetric objects (symmetric around a vertical axis like the letters “A” and “V”) more quickly than asymmetric objects.They also prefer objects that are smooth rather than rough, complex rather than simple, and have high-contrast contours, curves, and concentricity. Faces generally have smooth skin punctuated by high-contrast elemental objects. The elemental objects all exhibit strong lateral symmetry, high-contrast curves, and a high degree of concentricity (as does the whole face).

In this respect, the face might be regarded as a veritable treasure trove of pleasure-inducing stimuli. For the newborn, even an unfamiliar face is pleasure-inducing because it stimulates multiple core features across multiple sensory domains (for example, touch, audition, and vision) that its experience-expectant brain requires for normal development. Clearly, the pleasure experienced by a newborn looking into a caregiver’s face only increases over time as it comes to recognize this person and bond with her or him. Before this occurs, however, there must be a neural mechanism that increases the likelihood that the newborn spends more time looking at faces rather than, say, knees. The face provides a hedonic wonderland for the newborn, since a single experience can stimulate developing visual, somatic (touch), and auditory cortical regions in an integrated manner. As we saw in chapter 7, newborns also prefer certain forms of auditory stimulation, such as sounds that have slowly increasing and decreasing pitch contours—the singsong melody of motherese. Motherese, of course, emanates from mouths embedded in faces. Such pleasurable sounds serve to draw the attention of the newborn to the face, where they can be grouped together with other pleasurable features.

Not only do infants find faces pleasurable, they also differentiate attractive versus unattractive faces in much the same way as adults. In a series of compelling studies, psychologist Judith Langlois and her colleagues at the University of Texas at Austin found that infants as young as two months old prefer to look at attractive faces more than unattractive faces as rated independently by adults. Langlois and her colleagues began their study by taking a large collection of color slides of female faces to groups of undergraduate men and women and asking them to rank each slide based on its attractiveness from 1 (least attractive) to 5 (most attractive).The female faces were posed with a neutral emotional expression and glasses removed. Moreover, all clothing was masked so that judgments could be made based on facial features alone. There was remarkably high agreement across raters on attractiveness (0.97 coefficient alpha). Two groups were formed based on the rankings, eight slides with the highest attractiveness rating and eight slides with the lowest rating.

To determine preferences, infants were seated in their mothers’ laps and shown a series of two slides positioned side by side. One slide was from the most attractive group, the other from the least attractive group. Recordings were made of the amount of time each infant spent looking at the different faces. Differential fixation time is a common metric for evaluating preferences in infants. This metric generalizes in that children and adults also look longer at self- and independently rated attractive faces than at unattractive faces. Langlois found that infants spent significantly more time gazing at faces from the most attractive group than at faces from the least attractive group. This result has since been replicated and extended to show that infants also prefer more attractive faces to lesser attractive faces within other groups, including both male and female adult Caucasians, male adult African Americans, and other infants.

Needless to say, this finding was not consistent with the prevailing theories of child development at the time. Before Langlois began her research, many researchers assumed that preferences for attractiveness were based on the gradual learning of standards within a culture through a variety of sources (for example, media and social experiences) and that these emerged much later during development. Her data are at odds with this view, since these infant subjects have presumably had very limited exposure to such forces, in that they are preverbal and only two months old. Rather, these findings suggest that preferences for attractiveness are either in place at birth or shortly thereafter.

At present, many studies have shown remarkable agreement across raters from different age, gender, and cultural groups in terms of facial attractiveness rankings. Cross-cultural studies have been done with people in the United States, throughout Europe, China, Korea, South America, and Asia using multicultural faces, with fairly consistent agreement on which faces are the most attractive. A major question that is the focus of much investigation is what features humans use to make determinations about attractiveness and why,

Studies by evolutionary biologists and psychologists have found that people sometimes find the average attractive. Take a thousand faces, average their spatial characteristics, and you get a new face representing the group norm, which is generally rated to be slightly above average in attractiveness. In his classic text on the evolution of human sexuality, anthropologist Donald Symons hypothesized that natural selection drives adaptations, where at the population level, the optimum value of some trait is likely to be the mean. In this respect, a preference for the average value of a trait such as facial characteristics would be wise, since it would push the chooser toward mates with optimally adapted facial traits for things such as breathing, chewing, and any other functions linked genetically to the development of facial features. Randy Thornhill and psychologist Steve Gangestad theorized that a preference for averageness occurs because for some heritable traits, the mean value represents maximal genetic heterozygosity (allelic diversity).

Other studies have found that people tend to focus on secondary sex characteristics when making judgments of attractiveness. In humans and many other species, hormonal changes occurring at puberty can actually handicap an individual. For instance, increased testosterone production in adolescent males leads to increased musculature and energy expenditure. These changes, in turn, raise the metabolic demands of the body and draw resources away from other systems, including immune functioning. Since testosterone production creates a draw on immune function, it may be less costly for genetically fit males to have high levels of testosterone than for those more susceptible to environmental and/or genetic perturbations (for example, pathogens, maladaptive environmental conditions, genetic mutations). Likewise, increased estrogen production in pubertal girls indicates reproductive potential and fertility. Reproductive effort places increased metabolic demands on a woman’s body and draws resources away from other important biological processes, including immune function. Hence, secondary sexual characteristics of the face that become exaggerated at puberty (such as an extension of the lower jaw and broadening of the brow ridge in males) may be honest signals of phenotypic and genotypic condition.

In addition to averageness and attention to secondary sexual characteristics, many studies have found a strong relationship (perhaps not surprisingly) between ratings of facial attractiveness and fluctuating asymmetry of the face. Indeed, as mentioned earlier, in their extensive review, Anders Moller and Randy Thornhill found that in most species, the correlation between fluctuating asymmetry and mating success was strongest for body parts that are secondary sexual characteristics. The face is one such body part, since, as we have seen, it is replete with secondary sexual characteristics that become differentiated at puberty.

Numerous studies have demonstrated that both facial and body symmetry involving secondary sexual characteristics are related to reproductive success and health in general. In a diverse range of species tested, increased fluctuating asymmetry has been shown to be related to decreased fecundity, growth rate, survival, and metabolic efficiency. For instance, increased fluctuating asymmetry in men has been shown to be related to a number of fertility measures. Population biologist John Manning and his colleagues from the University of Liverpool studied males referred to a reproductive medicine clinic at a local hospital for routine semen analysis.They found that men with greater body asymmetry had fewer numbers of sperm per ejaculate, lower sperm speed, and reduced sperm migration relative to their more symmetric counterparts. In women, breast asymmetry has been found to correlate negatively with fecundity and to the probability of marriage.

Other studies have shown that body symmetry is related to how efficiently our bodies use energy. If one has a normally low metabolic cost associated with the maintenance of body processes, this should theoretically free energy for use in other ways, such as maximizing developmental homeostasis. Consistent with this ideal, Manning’s group has also found that people with greater body symmetry exhibit a lower resting metabolic rate, measured as oxygen consumption at rest.

The relationship between overall physiological health and body symmetry has also been examined by several investigators. Gangestad and Thornhill gave a series of health questionnaires to 203 romantically involved couples where both the man and the woman rated self and partner. The questions asked about the individual’s current condition in eight specific physical health domains, including muscularity, energy, stamina, vigorousness, robustness, lethargy, physical tightness, and cardiovascular fitness. The researchers found a significant negative correlation between a composite physicality score (summed from the individual domain scores) and body asymmetry. As asymmetry increased, perceived general physical health decreased.

Other investigators have found similar results when focusing just on facial asymmetry. In a study of 101 college students, evolutionary psychologists Todd Shackleford and Randy Larsen examined the relationship between facial asymmetry and a broad of range of physical, emotional, and psychological health indicators. Head shots of students were taken and used to extract estimates of facial asymmetry using bilateral measurements at the outer eye, inner eye, nostril width, cheekbone width, and jaw width. Each subject was also given an extensive battery of health questionnaires concerning mood and emotional state, personality, life orientation, sociability, impulsivity, and general symptomatology. Additionally, subjects were requested to complete reports on their daily activities, moods, and physical symptoms twice each day. These included standard physical symptoms such as headaches, trouble concentrating, runny nose, sore throats and coughs, gastrointestinal problems, and so forth. Finally, each participant had his or her overall aerobic fitness assessed by measuring cardiac recovery time following a standard exercise protocol to raise their heart rates to at least thirty beats per minute beyond resting state.

A separate set of college-age observers were asked to rate each photograph of the subjects along a number of dimensions including attractiveness, happiness, reliability, agreeableness, intelligence, emotional stability, activeness, and others. The overall results of the study painted a complex picture revealing how variation in facial asymmetry impacts several key aspects of daily life in the bearer and how others view them.

In terms of physical health, subjects with greater facial asymmetry were more likely than their symmetric counterparts to report negative physical symptoms, such as backache, muscle soreness, reduced vigor, and trouble concentrating. They were also more likely to complain of depression, perform more impulsive acts, and view their lives as being outside of their personal control. Men with greater facial asymmetry, in particular, tended to score higher on measures of mania and schizophrenia components of the Minnesota Multiphasic Personality Inventory (MMPI), a classic clinical assessment, than those who were more symmetric. Consistent with this observation, several studies have now shown that schizophrenia in adults and hyperactivity disorder in boys are both associated with increased body and facial fluctuating asymmetry.

On the flip side, individuals with relatively more facial symmetry when compared to their counterparts were more likely to be optimistic, view themselves as superior, and score higher on measures of narcissism. Interestingly, extroversion seems to correlate positively with the degree of facial asymmetry in women and negatively in men. Women with more facial asymmetry tend to be more extroverted, while men tend to be more introverted.

Independent judgments by external observers were also systematically related to the degree of facial asymmetry in the subjects. Individuals with greater facial symmetry were viewed by others as being more conscientious, intelligent, active, agreeable, and genuine than those with less symmetry.This study confirms other reports that facial asymmetry correlates with a number of important markers of physiological, emotional, and psychological health.

Symmetry Signals and Pleasure

Thus far we have found that increased body and facial asymmetry are associated with decreases in certain indicators of fertility, as well as self-reported physical, emotional, and psychological well-being. But does greater facial symmetry translate into improved mating success? In other words, do people actually have a preference for symmetric mates?

Symmetry is a basic property that is preferred and sought after by newborns and toddlers alike. They recognize more quickly and exhibit greater pleasure in viewing symmetric objects than those that are asymmetric. I have shown in earlier chapters that an anatomical and developmental imperative exists for symmetry-seeking almost immediately after birth, since it represents an optimal form of stimulation for experience-expectant maturation of the primary visual cortex and additional downstream visual areas (for example, V2, V3, and inferotemporal cortex).

The preference for symmetry right after birth would be expected to continue through childhood, since synaptic pruning of these visual cortical areas continues for decades.This early preference thus forms the basis for a fondness of symmetry in adults. Moreover, since body and facial symmetry are related to general health (and are, hence, potential fitness indicators), an inborn preference for symmetry that mediates normal brain growth and development would be a perfect trait to be co-opted through sexual selection mechanisms. If this scenario is true, one would predict all six of the following conditions to be true as well.

Condition 1 The preference for symmetry is expressed at or very near birth. Cell proliferation in the primary visual cortex is maximal at about the time that infants begin to take pleasure from highly symmetric objects. Stimulation of V1 and related downstream visual cortical areas is required for normal brain growth and development, including synaptic pruning (see chapter 8). Laterally symmetric objects represent an ideal form of stimulation during this developmental period, since they have redundant spatial information. Such redundancy may facilitate the increased recognition speed that infants exhibit in processing symmetric relative to asymmetric objects.

Condition 2 The preference for symmetry generalizes across many object forms. This condition is critical to the hypothesis that a preference for symmetry originated as a mechanism for ensuring normal brain development and maturation independent of its co-option as a fitness indicator through sexual selection. If a fondness for symmetry in newborns is only fulfilling a role as a potential fitness indicator, we would expect to see the preference emerge strictly in relation to bodies and faces—those objects to which fitness most applies. On the other, if the preference for symmetry in newborns is first and foremost a mechanism to facilitate brain growth, one would expect that all types of symmetric objects are preferred to their asymmetric counterparts. Indeed, the data show the latter to be true, since newborns merely a few months old prefer a wide range of symmetric objects to their asymmetric versions. Such objects include abstract drawings, solid geometric forms, landscape features, faces, and many others.

Condition 3 Objects that have multiple markers of symmetry, such as faces, are especially pleasurable. If a general preference for symmetrical features exists independent of the form in which they appear, objects that have multiple salient points that exaggerate this condition should be especially pleasurable to view. Newborns tend to look longer (a proxy for preference) at symmetrically complex objects than at simpler symmetric objects. Newborns also prefer line drawings of faces with the proper symmetry of features preserved more than drawings where the features are shifted and symmetry is broken. Taken together, such evidence indicates that objects with multiple salient features that emphasize symmetry should be even more pleasurable and preferred to simpler, symmetric forms with fewer features.

Condition 4 Symmetry is a reliable marker of phenotypic quality. As we have just reviewed, body and facial symmetry are related to improved markers of fertility in both men and women. Both body and facial symmetry have also been shown to be related to better self-reported physical, emotional, and psychological health and well-being. Moreover, independent observers rank people with highly symmetric faces as more conscientious, agreeable, and intelligent, among other positive traits, compared to faces bearing less symmetry. Hence there is evidence that individuals with greater symmetry enjoy better health and fitness and that they are perceived by others as being more likely to possess certain positive personality traits in comparison to their less symmetric counterparts.These and other studies suggest that body and facial symmetry are reliable markers of phenotypic condition, and thus might be used as fitness indicators during mate selection.

Condition 5 Adults prefer symmetrical bodies and faces. If body and facial asymmetry are fitness indicators that are used during mate selection, individuals should be able to detect variation in symmetry and prefer it in a potential suitor. Consequently, everything else being equal, individuals with greater body and facial symmetry should be seen as more attractive than those who are less symmetrical. By extension, individuals with the greatest body and facial symmetry (again, all else being equal) should have greater mating success than their asymmetric counterparts.

Condition 6 Adults prefer symmetrical to asymmetrical versions of most objects in general, even those unrelated to faces and bodies. If a preference for symmetry emerges in newborns to guide proper brain development through experience-expectant mechanisms, such stimulation need not be in one specific form or another. Faces are excellent forms of stimulation in this scenario since, as we have seen, they possess many cardinal features that activate the pleasure instinct and facilitate neural development. But all symmetric objects should facilitate this process to some degree. Hence, if the preference for symmetry in newborns and children is carried over to adulthood, we would expect it to generalize to other object forms beyond faces and bodies. Let us now turn to a discussion of the evidence for the final two conditions.

Do people have a preference for symmetric mates, and if so, how does this preference impact their selections? In a landmark study, evolutionary psychologist David Buss and his colleagues interviewed more than ten thousand individuals from thirty-seven different cultures about their mating preferences. While men tended to value physical attractiveness slightly more than women in selecting a mate, both sexes from virtually every culture studied ranked appearance as one of the most important factors overall. As we have already seen, there is also remarkable agreement across cultures on aesthetic judgments of facial attractiveness, and even two-month-old infants seem capable of making the distinction. So are people with greater symmetry seen as more attractive?

Perhaps no other research area in evolutionary psychology has received more attention from the media as the study of attractiveness. Several original and replication studies have shown that in general we find potential mates with greater body and facial symmetry to be most sexually attractive. For instance, in comparison to men with high asymmetry, symmetrical men have greater facial attractiveness (as rated by both male and female observers), more sexual partners during a lifetime, and an increased frequency of sexual affairs with partners outside of their primary relationship, begin to have sex earlier in life, and produce disproportionately more copulatory female orgasms in their partners.

In a recent study, biologist Craig Roberts and colleagues from the University of Newcastle found that facial asymmetry and attractiveness correlate positively with each other and with a key marker of immune function. The investigators showed fifty women (aged eighteen to forty-nine years old) color head shots of men with a neutral facial expression. As in similar studies, all photographs were digitally masked so just the face was visible. The women were asked to rank the attractiveness of each face using a seven-point scale. The investigators extracted a composite estimate of fluctuating asymmetry by measuring symmetry at seven distinct bilateral facial landmarks. In an interesting twist, blood samples were also collected from the men and genotyped for heterozygosity at key loci in the major histocompatibility complex (MHC—see chapter 5). Recall from chapter 5 that the MHC genes code for immune cells that identify intruding disease organisms, thus functioning as our immune system’s first line of defense. Of importance in this discussion is the fact that MHC genes have upward of a hundred or so different alleles, each providing immunity against different sets of potential disease strains.The level of zygosity refers to how often a specific allele is repeated at different loci within the MHC. It is believed that greater allelic diversity in the MHC leads to a broader resistance to different pathogens. Individuals vary considerably in their degree of heterozygosity, with most people being homozygous at a few alleles. Roberts and his colleagues found that the degree of heterozygosity at key loci in the MHC correlated positively with fluctuating asymmetry in the male faces. Men with greater heterozygosity—who are presumably equipped to fight off a greater variety of invading pathogens than their more homozygous counterparts—had the most symmetrical faces. Moreover, the women rated these faces as the most attractive.

Nonfacial secondary sexual characteristics have also been found to be linked to ratings of attractiveness. As we saw earlier, waist-to-hip ratios in women of approximately .7 are seen by both men and women as most attractive. In other experiments, Devendra Singh demonstrated that breast symmetry is positively correlated with men’s judgments of attractiveness as well as their interests in both short- and long-term relationships. Like faces and other secondary sexual characteristics, when observed across a population of individuals, fluctuating asymmetry of breasts tends to be large relative to absolute size (that is, absolute breast size asymmetry divided by breast size).Whereas most body parts exhibit fluctuating asymmetry of no more than 1 percent of the overall body part size, breast asymmetry tends to be closer to 5 percent of absolute size in all cultures that have been studied.

These data indicate that people are able to detect differences in body and facial symmetry, and use this information to guide their choice of potential mates. It is interesting that when asked to define what makes a person attractive, people often say something about a particular look or a particular body part (for example, the eyes). Evidence shows that we use symmetry as an important metric in defining attractiveness and identifying preferred mates, even though we may not consciously recognize this implicit calculation. Interestingly, we use mental calculations of symmetry every day in many other contexts, such as in our appreciation of art, choices of jewelry and clothing, and what consumer products to buy. Let us now examine the broader way the pleasure of symmetry impacts our adult lives.

Symmetry and Aesthetics: An Example of a General Process

Adults are not only drawn to symmetric mates. As would be predicted by the theoretical perspective being discussed throughout this book, the pleasure we take from seeing highly symmetric objects extends beyond bodies and faces. For instance, similar to newborns, adults are able to recognize and process vertically symmetric objects more quickly than objects with similar features that are not symmetric. Moreover, symmetric objects and patterns are preferred by adults over asymmetric versions even if they do not serve any apparent biological function (for example, such as mate selection). Indeed, there is widespread use of symmetric designs for decorative art among cultures diverse in region, ethnicity, and time.

In a series of interesting studies, psychologist Lauren Harris adopted classic abstract designs seen in different cultures (for example, Aonikenk, Navajo, and Yoruba) and manipulated them in terms of their symmetry. In one condition, the geometric shape was manipulated such that two versions of the same form were presented to adult subjects—one perfectly symmetrical, the other asymmetrical. The subjects were asked to “choose the design that is more attractive in each pair of designs.” In a second condition, both object shape and coloration were varied—symmetrical in one object and asymmetrical in the comparison. Finally, in the third condition, objects varied in the orientation of their symmetry—vertical versus at forty-five degrees.This third condition was included to replicate previous work that has shown that both newborns and adults recognize objects with vertical symmetry more quickly than other orientations.

Harris and her colleagues found that in all comparisons the symmetric version was seen as being more attractive than the asymmetric counterpart. This suggests that symmetry is preferred in nonbiological signals outside the context of fitness, and that the preference is robust since it occurred with respect to the primary features of shape, color, and orientation. Condition two, in which both color and shape symmetry were varied, was found to have the largest effect size (differences in the mean number of designs chosen as most attractive in each condition) of all the conditions. This finding is exactly what would be predicted from the theory being considered, namely that there are several core preferences (across all sensory domains) that emerge during development that facilitate normal brain growth and maturation. The pleasure instinct prods us to seek these basic stimulus forms to fine-tune each sensory system to the environment in which the individual resides.These core features are additive in the sense that objects with multiple pleasure-inducing stimulus forms should be preferred to those objects with fewer forms. Hence it is more pleasurable to look at a real face (in both newborns and adults) with smooth skin, high contrast and concentricity, and symmetry than a simple line drawing of a face with just symmetry. In this case we see that color symmetry acts additively with shape symmetry to produce an even greater preference than either would have alone.

Interestingly, Harris and her colleagues also examined the impact of symmetric and asymmetric facial painting on overall facial attractiveness, noting that such practices are common across geographically diverse tribal societies (for example, the Selk’nam of South America, the Huli of Papua New Guinea, the Kikuyu of Africa, and Blackfoot Indians of North America). Again, they had three conditions. In the first condition, unpainted symmetric versions of a face were compared against unpainted asymmetric versions of the same face. In the second condition, symmetric faces with laterally symmetrical paint designs were compared against asymmetric faces with laterally asymmetric designs. In the final condition, symmetrical faces with asymmetrical paint were compared to asymmetric faces with symmetric paint. Subjects were asked to “choose the face that is physically more attractive in each pair of faces.”

The researchers found that symmetric faces with symmetric paint were viewed as the most attractive in terms of absolute preference across all of the conditions. They also found, as expected, that unpainted symmetric faces were preferred to unpainted asymmetric faces. Interestingly, the application of an asymmetric design to a symmetric face decreased its attractiveness, while the application of a symmetric design increased the attractiveness of asymmetric faces. This pattern of results indicates that additional symmetry features that have no association with overall fitness or phenotypic quality have an additive effect on preference ratings of biologically relevant features that do indicate fitness.

Our innate preferences for symmetry and proportion that impact everyday behaviors are but two simple spatial examples of what I believe represent a general process. As mentioned in earlier chapters, the pleasure instinct creates strong preferences for distinct stimulus features in every sensory domain. The developing brain is thirsty for particular experiences that optimize both synaptogenesis and synaptic pruning. Some feature preferences crafted by the pleasure instinct are likely carried into adulthood unaltered and continue to steer our everyday behaviors and choices in subtle and not so subtle ways (for example, our love of sugars and fats). But my sense is that many of the preferences that facilitate brain development (arguably adaptations driven by natural selection) have also been amplified at some point in our phylogenetic history by sexual selection processes. In the next chapter we will discuss a temporal example and examine the complicated manner in which our preference for repetition and rhythm is expressed in our everyday lives.