The Bell Curve: Intelligence and Class Structure in American Life - Richard J. Herrnstein, Charles Murray (1996)
Part I. The Emergence of a Cognitive Elite
Chapter 4. Steeper Ladders, Narrower Gates
Cognitive partitioning through education and occupations will continue, and there is not much that the government or anyone else can do about it. Economics will be the main reason. At the same time that elite colleges and professional schools are turning out brighter and brighter graduates, the value of intelligence in the marketplace is rising. Wages earned by people in high-IQ occupations have pulled away from the wages in low-IQ occupations, and differences in education cannot explain most of this change.
Another force for cognitive partitioning is the increasing physical segregation of the cognitive elite from the rest of society. Members of the cognitive elite work in jobs that usually keep them off the shop floor, away from the construction site, and close to others who also tend to be smart. Computers and electronic communication make it increasingly likely that people who work mainly with their minds collaborate only with other such people. The isolation of the cognitive elite is compounded by its choices of where to live, shop, play, worship, and send its children to school.
Its isolation is intensified by an irony of a mobile and democratic society like America’s. Cognitive ability is a function of both genes and environment, with implications for egalitarian social policies. The more we succeed in giving every youngster a chance to develop his or her latent cognitive ability, the more we equalize the environmental sources of differences in intelligence. The irony is that as America equalizes the circumstances of people’s lives, the remaining differences in intelligence are increasingly determined by differences in genes. Meanwhile, high cognitive ability means, more than ever before, that the chances of success in life are good and getting better all the time. Putting it all together, success and failure in the American economy, and all that goes with it, are increasingly a matter of the genes that people inherit.
Add to this the phenomenon known as assortative mating. Likes attract when it comes to marriage, and intelligence is one of the most important of those likes. When this propensity to mate by IQ is combined with increasingly efficient educational and occupational stratification, assortative mating by IQ has more powerful effects on the next generation than it had on the previous one. This process too seems to be getting stronger, part of the brew creating an American class system.
As Mae West said in another context, goodness has nothing to do with it. We are not talking about what should have been but what has been. The educational system doessort by cognitive ability at the close of the twentieth century in a way that it did not at the opening of the century. The upper strata of intelligence are being sucked into a comparatively few occupations in a way that they did not used to be. Cognitive ability is importantly related to job productivity. All of these trends will continue under any social policy. We are optimistic enough to believe that no administration, Left or Right, is going to impede the education of the brightest, or forbid the brightest from entering the most cognitively demanding occupations, or find a way to keep employers from rewarding productivity. But we are not so optimistic that we can overlook dark shadows accompanying the trends.
To this point, we have avoided saying what social consequences might be expected. This omission has been deliberate, for part of a candid answer must be, “We aren’t sure.” We can be sure only that the trends are important. Cognitive stratification as a central social process is something genuinely new under the sun. One of our purposes is to bring it to public attention, hopeful that wisdom will come from encouraging more people to think about it.
It is impossible to predict all the ways in which cognitive stratification will interact with the workings of an American democracy that is in flux. We do have some thoughts on the matter, however, and in this chapter use the available scientific data to peer into the future. The data center on the dynamics that will make cognitive stratification more pronounced in the years to come—the differences greater, the overlap smaller, the separation wider. We reserve our larger speculations about the social consequences for Chapters 21 and 22.
THE CHANGING MARKET FOR ABILITY
The overriding dynamic that will shape the effects of cognitive stratification is the increasing value of intelligence in the marketplace. The smart ones are not only being recruited to college more efficiently, they are not only (on average) more productive in the workplace, their dollar value to employers is increasing and there is every reason to believe that this trend will continue. As it does so, the economic gap separating the upper cognitive classes from the rest of society will increase.
The general shape of what has been happening is shown in the figure for a representative high-IQ occupation, engineering, compared to the average manufacturing employee, starting back in 1932. As always, dollar figures are expressed in 1990 dollars. The 1950s turn out to have been the decade of hidden revolution for income, just as it was for education and status. Throughout the 1930s and 1940s, the average engineer and the average manufacturing employee remained in roughly a constant economic relationship, even converging slightly. Then from 1953 to 1961 the average engineer’s salary nearly doubled while the manufacturing employee’s salary followed the same gradually rising trend and increased by only 20 percent. By the end of the 1980s, the average manufacturing employee had to get by on about $23,000 a year while the engineer made an average of $72,000. The difference in their purchasing power had tripled since the 1940s, which is enough to put them in separate economic brackets.
Engineers’ salaries as an example of how intelligence became much more valuable in the 1950s
Source: U.S. Bureau of the Census 1975, Tables D802-810, D913-926; Bureau of Labor Statistics 1989, Tables 80, 106.
Were the 1980s Good or Bad for Income?
There are half a dozen different ways to view the economy during the 1980s. Because most of it fell in Ronald Reagan’s presidency, an intense political struggle to characterize the decade as economically “good” or “bad” has ensued. The main source of confusion lies in the distinction between household income, which went up for all income groups, driven by the increase in two-income families and low unemployment, and real wages, which (generally) rose for white-collar workers and fell for blue-collar workers. There are also confusions that arise because the value of benefit packages rose even though cash wages did not and because of controversies over the proper calculation of changes in real purchasing power. We will not try to adjudicate these issues or the role that President Reagan’s economic policies played, which have taken whole books to argue out.
The comparison between engineers and manufacturing employees is a microcosm of what has happened generally to American workers. Using data from the Current Population Surveys, economists Lawrence Katz and Kevin Murphy, among others, have established that from 1963 to 1987, male workers making the highest 10 percent of wages enjoyed a rise of about 40 percent, while the real wages of those at the corresponding low end were close to static.1
We opened the chapter by asserting that cognitive ability has been a key factor in this process. Next we look at the reasons for this conclusion.
The Role of Education
The standard way of interpreting the figure for engineers and manufacturing is to talk about education. During the last quarter-century, real wages rose more than twice as much for workers with college educations than for those with high school or less.2 Trends were not uninterrupted within the interval. Following the huge expansion of the post-World War II college population, it seemed for a while that the economic benefits of education were being swamped by oversupply, as wages fell during the 1970s for college-educated people.3 But in the 1980s, the trend reversed. Real wages for highly educated people started once again to climb and wages fell for those with twelve or fewer years of schooling.4
The table below gives the percentage change in real wages for fulltime male workers5 at three educational levels during the 1980s, broken out by whether they are new workers (one to five or twenty-six to thirty-five years of work experience). The dramatic changes occurred among young men just coming into the labor market. High school graduates and dropouts saw their real wages plunge, while young men with college educations enjoyed a healthy increase.6 Meanwhile, experienced older men saw little real change in income whatever their level of education. Why the difference between the age groups? Interpretively, wages for men with many years of experience reflect their work history as well as their immediate economic value. Wages for people just entering the labor force are more purely an expression of prevailing market forces. The job market reevaluated schooling during the past two decades: Educated workers, having been devalued in the 1970s, became increasingly valuable in the 1980s, in comparison with less educated workers.7
Education, Experience, and Wages, 1979-1987
Source: Adapted from Katz and Murphy, 1990, Table 1.
Percentage Change in Wages
New workers (1-5 years of experience)
Less than 12 years of school
High school degree
16 or more years of school
Old workers (26-35 years of experience)
Less than 12 years of school
High school degree
16 or more years of school
Why have the economic returns to education lately risen, thereby widening the income gap between the educated and the uneducated? Perhaps, say some commentators, the wage inequality problem is technological, as machines displace people from low-skill jobs. Perhaps schools are failing to teach people skills that they used to teach, or maybe the schools are doing as well as ever but the blue-collar jobs that require only low-level skills are emigrating to countries where labor is cheaper, thereby creating an oversupply of less educated workers in America. Perhaps the welfare system is eroding the need to work among the low-skill population, or the weakening labor unions are not protecting their economic interests, or a declining real minimum wage is letting the wage structure sag at the low end.
These possibilities all bear on a crucial issue: How much good would it do to improve education for the people earning low wages? If somehow the government can cajole or entice youths to stay in school for a few extra years, will their economic disadvantage in the new labor market go away? We doubt it. Their disadvantage might be diminished, but only modestly. There is reason to think that the job market has been rewarding not just education but intelligence.8
The Mysterious Residual
The indispensable database for analyzing wages over time is the Current Population Survey, the monthly national survey conducted by the Bureau of the Census and the Bureau of Labor Statistics, which asks people only about their years of education, not their IQs. But as the sophisticated statistical analyses of wage variation have accumulated, experts have come to agree that something beyond education, gender, and experience has been at work to increase income disparities in recent times.9 The spread in real wages grew between 1963 and 1987 even after taking those other factors into account.10 The economic term for this unexplained variation in wages is “the residual.”11
To understand the growing wage inequality requires an account of this residual variation. Residual wage variation for both men and women started rising in about 1970 and seems still to be rising. Among economists, there is a consensus that, whatever those residual characteristics consist of, it has been mainly the demand for them, not their supply, that has been changing and causing increasing wage inequality for a generation, with no signs of abating.12Despite the public focus on the increasing importance of education in the workplace, most of the increasing wage inequality during the past two and a half decades is due to changes in the demand for the residual characteristics of workers rather than to changes in the demand for education or experience.13 The job market for people lacking the residual characteristics declined, while expanding for people having them.
The Case for IQ as the Residual
What then is this residual, this X factor, that increasingly commands a wage premium over and above education? It could be a variety of factors. It could be rooted in diligence, ambition, or sociability.14 It could be associated with different industries or different firms within industries, or different wage norms (e.g., regional variations, variations in merit pay), again insofar as they are not accounted for by the measured variables. Or it could be cognitive ability. Conclusive evidence is hard to come by, but readers will not be surprised to learn that we believe that it includes cognitive ability. There are several lines of support for this hypothesis.
As a first cut at the problem, the changing wages have something to do with the shifting occupational structure of our economy. High-status, and therefore relatively high-paying, jobs are tipped toward people with high intelligence, as Chapter 2 showed. As the high-end jobs have become more numerous, demand must rise for the intellectual abilities that they require. When demand rises for any good, including intelligence, the price (in this case, the wages) goes up. Purely on economic grounds, then, wage inequality grew as the economic demand for intelligence climbed.
We further know from the data discussed in Chapter 3 that cognitive ability affects how well workers at all levels do their jobs. If smarter workers are, on average, better workers, there is reason to believe that income within job categories may be correlated with intelligence.
Still further, we know that the correlation between intelligence and income is not much diminished by partialing out the contributions of education, work experience, marital status, and other demographic variables.15 Such a finding strengthens the idea that the job market is increasingly rewarding not just education but intelligence.
Finally, McKinley Blackburn and David Neumark have provided direct evidence in their analysis of white men in the National Longitudinal Survey of Youth (NLSY). Education and intelligence each contributed to a worker’s income, but the smart: men earned most of the extra wage benefit of education during the past decade.16 The growing economic benefits of either schooling or intelligence are disproportionately embodied in the rising income of educated people with high test scores and in the falling wages earned by less educated people with low scores.17
This premium for IQ applies even within the high-IQ occupations that we discussed in Chapter 2. In the NLSY, among people holding one of these jobs, the 1989 weekly earnings (expressed in 1990 dollars) of those in the top 10 percent of IQ were $977, compared to $697 for those with IQs below the top 10 percent, for an annual income difference of over $14,000.18 Even after extracting any effects of their specific occupations (as well as of the differing incomes of men and women), being in the top 10 percent in IQ was still worth over $11,000 in income for those in this collection of prestigious occupations.
Why Cognitive Ability Has Become More Valuable to Employers
This brings us as far as the data on income and intelligence go. Before leaving the topic, we offer several reasons why the wage premium for intelligence might have increased recently and may be expected to continue to increase.
Perhaps most obviously is that technology has increased the economic value of intelligence. As robots replace factory workers, the factory workers’ jobs vanish, but new jobs pop up for people who can design, program, and repair robots. The new jobs are not necessarily going to be filled by the same people, for they require more intelligence than the old ones did. Today’s technological frontier is more complex than yesterday’s. Even in traditional industries like retailing, banking, mining, manufacturing, and farming, management gets ever more complex. The capacity to understand and manipulate complexity, as earlier chapters showed, is approximated by g, or general intelligence. We would have predicted that a market economy, faced with this turn of events, would soon put intelligence on the sales block. It has. Business consultancy is a new profession that is soaking up a growing fraction of the graduates of the elite business schools. The consultants sell mainly their trained intelligence to the businesses paying their huge fees.
A second reason involves the effects of scale, spurred by the growth in the size of corporations and markets since World War II A person who can dream up a sales campaign worth another percentage point or two of market share will be sought after. What “sought after” means in dollars and cents depends on what a point of market share is worth. If it is worth $500,000, the market for his services will produce one range of salaries. If a point of market share is worth $5 million, he is much more valuable. If a point of market share is worth $100 million, he is worth a fortune. Now consider that since just 1960, the average annual sales, per corporation, of America’s five hundred largest industrial corporations has jumped from $1.8 billion to $4.6 billion (both figures in 1990 dollars). The same gigantism has affected the value of everything from the ability to float successful bond offerings to the ability to negotiate the best prices for volume purchases by huge retail chains. The magnitude of the economic consequences of ordinary business transactions has mushroomed, and with it the value of people who can do their work at a marginally higher level of skill. All the evidence we have suggests that such people have, among their other characteristics, high intelligence. There is no reason to think that this process will stop soon.
Then there are the effects of legislation and regulation. Why are certain kinds of lawyers who never see the inside of a courtroom able to command such large fees? In many cases, because a first-rate lawyer can make a difference worth tens of millions of dollars in getting a favorable decision from a government agency or slipping through a tax loophole. Lawyers are not the only beneficiaries. As the rules of the game governing private enterprise become ever more labyrinthine, intelligence grows in value, sometimes in the most surprising places. One of our colleagues is a social psychologist who supplements his university salary by serving as an adviser on jury selection, at a consulting fee of several thousand dollars per day. Based on his track record, his advice raises the probability of a favorable verdict in a liability or patent dispute by about 5 to 10 percent. When a verdict may represent a swing of $100 million, an edge of that size makes him well worth his large fee.
We have not exhausted all the reasons that cognitive ability is becoming more valuable in the labor market, but these will serve to illustrate the theme: The more complex a society becomes, the more valuable are the people who are especially good at dealing with complexity. Barring a change in direction, the future is likely to see the rules for doing business become yet more complex, to see regulation extend still further, and to raise still higher the stakes for having a high IQ.
The End Result: Prosperity for Those Lucky Enough to Be Intelligent
After all that has gone before, it will come as no surprise to find that smart people tend to have high incomes. The advantage enjoyed by those who have high enough IQs to get into the high-IQ occupations is shown in the figure below. All of the high-IQ occupations have median wages well out on the right-hand side of the distribution.19 Those in the top range of IQ had incomes that were conspicuously above those with lower IQs even within the high-IQ occupations. The overall median family income with a member in one of these occupations and with an IQ in the top 10 percent was $61,100, putting them at the 84th percentile of family incomes for their age group. These fortunate people were newly out of graduate school or law school or medical school, still near the bottom of their earnings trajectory as of their early thirties, whereas a large proportion of those who had gone into blue-collar jobs (disproportionately in the lower IQ deciles) have much less room to advance beyond this age.20 In other words, the occupational elite is prosperous. Within it, the cognitive elite is more prosperous still.
The high-IQ occupations also are well-paid occupations
Source: U.S. Department of Labor 1991.
Income as a Family Trait
America has taken great pride in the mobility of generations: enterprising children of poor families are supposed to do better than their parents, and the wastrel children of the rich are supposed to fritter away the family fortune. But in modern America, this mobility has its limits. The experts now believe that the correlation between fathers’ and sons’ income is at least .4 and perhaps closer to .5.21 Think of it this way: The son of a father whose earnings are in the bottom 5 percent of the distribution has something like one chance in twenty (or less) of rising to the top fifth of the income distribution and almost a fifty-fifty chance of staying in the bottom fifth. He has less than one chance in four of rising above even the median income.22 Economists search for explanations of this phenomenon in structural features of the economy. We add the element of intellectual stratification. Most people at present are stuck near where their parents were on the income distribution in part because IQ, which has become a major predictor of income, passes on sufficiently from one generation to the next to constrain economic mobility.
COGNITIVE SORTING THROUGH PHYSICAL SEPARATION
The effects of cognitive sorting in education and occupation are reified through geography. People with similar cognitive skills are put together in the workplace and in neighborhoods.
Cognitive Segregation in the Workplace
The higher the level of cognitive ability and the greater the degree of homogeneity among people involved in that line of work, the greater is the degree of separation of the cognitive elite from everyone else. First, consider a workplace with a comparatively low level of cognitive homogeneity—an industrial plant. In the physical confines of the plant, all kinds of abilities are being called upon: engineers and machinists, electricians and pipefitters and sweepers, foremen and shift supervisors, and the workers on the loading dock. The shift supervisors and engineers may have offices that give them some physical separation from the plant floor, but, as manufacturers have come to realize in recent years, they had better not spend all their time in those offices. Efficient and profitable production requires not only that very different tasks be accomplished, using people of every level of cognitive ability, but that they be accomplished cooperatively. If the manufacturing company is prospering, it is likely that a fair amount of daily intermingling of cognitive classes goes on in the plant.
Now we move across the street to the company’s office building. Here the average level of intelligence is higher and the spread is narrower. Only a handful of jobs, such as janitor, can be performed by people with low cognitive ability. A number of jobs can be done by people of average ability—data entry clerks, for example. Some jobs that can be done adequately by people with average cognitive ability turn into virtually a different, and much more important, sort of job if done superbly. The job of secretary is the classic example. The traditional executive secretary, rising through the secretarial ranks until she takes charge of the boss’s office, was once a familiar career path for a really capable, no doubt smart, woman. For still other jobs, cognitive ability is important but less important than other talents—among the sales representatives, for example. And finally there is a layer of jobs among the senior executives and in the R&D department for which cognitive ability is important and where the mean IQ had better be high if the company is to survive and grow in a competitive industry. In the office building, not only cognitive homogeneity has increased; so has physical separation. The executives do not spend much time with the janitors or the data entry clerks. They spend almost all their time interacting with other executives or with technical specialists, which means with people drawn from the upper portion of the ability distribution.
Although corporate offices are more stratified for intelligence than the manufacturing plant, some workplaces are even more stratified. Let’s move across town to a law firm. Once again, the mean IQ rises and the standard deviation narrows. Now there are only a few job categories—for practical purposes, three: secretaries, paralegals or other forms of legal assistants, and the attorneys. The lowest categories, secretarial and paralegal work, require at least average cognitive skills for basic competence, considerably more than that if their jobs are to be done as well as they could be. The attorneys themselves are likely to be, virtually without exception, at least a standard deviation above the mean, if only because of the selection procedures in the law schools that enabled them to become lawyers in the first place. It remains true that part of the success of the law firm depends on qualities that are only slightly related to cognitive skills—the social skills involved in getting new business, for example. And attorneys in almost any law firm can be found shaking their heads over the highly paid (and smart) partner who is coasting on his subordinates’ talents. But the overall degree of cognitive stratification in a good law firm is extremely high. And note an important distinction: It is not that stratification within the law firm is high; rather, the entire workplace represents a stratum highly atypical of cognitive ability in the population at large.
These rarefied environments are becoming more common because the jobs that most demand intelligence are increasing in number and economic importance. These are jobs that may be conducted in cloistered settings in the company of other smart workers. The brightest lawyers and bankers increasingly work away from the courtroom and the bank floor, away from all except the most handpicked of corporate clients. The brightest engineers increasingly work on problems that never require them to visit a construction site or a shop floor. They can query their computers to get the answers they need. The brightest public policy specialists shuttle among think tanks, bureaucracies, and graduate schools of public policy, never having to encounter an angry voter. The brightest youngsters launch their careers in business by getting an M.B.A. from a top business school, thence to climb the corporate ladder without ever having had to sell soap or whatever to the company’s actual customers. In each example, a specialized profession within the profession is developing that looks more and more like academia in the way it recruits, insulates, and isolates members of the cognitive elite.
As soon as a town grows larger than a few dozen households in size, it starts to develop neighborhoods. As towns become cities, this tendency becomes a reliable law of human communities. People seek out comfortable neighborhoods they can afford. For some people, this will mean looking for a particular kind of setting. Parents with young children typically want parks, good schools, and neighbors with young children. Single people in their twenties and thirties making good money often gravitate toward upscale urban neighborhoods with lots of places to go and things to do.
The result is to produce neighborhoods with a high level of socio-economic partitioning. The factory worker seldom lives next door to the executive, and this was as true in 1900 as in the last years of the century. The wealthy people have always been the most mobile. But in the late twentieth century, the most mobile people are increasingly drawn from the cognitive elite. In thinking about these changes, we will focus on their implications for the way that the children of the cognitive elite are raised, for therein lies one of the main potential sources of trouble.
First, the urbanization of the nation has meant that a much smaller proportion of the population grows up in places where socioeconomic mixing occurs naturally. Given a small enough town, there are not enough elementary schools to segregate the children efficiently. The children of the local upper crust may live on the street with the large houses, but there are not enough of them to fill up a whole school. After elementary school, every child in the town goes to the same middle school and high school. Such towns now constitute a shrinking proportion of the population, however. As of 1990, 78 percent of the overall population lived in metropolitan areas.23
Cognitive segregation is also being intensified by failures of government in large cities. As urban school systems deteriorate, people with money relocate to rich suburbs because that is where the good public school systems are; if they remain in the city, they send their children to private schools, which are even more homogeneous. As crime rates rise, people with money relocate to suburbs where the crime rates are low, or they concentrate ever more densely within the safer parts of the city. As urban tax rates rise, the middle class flees, leaving behind even more starkly segregated poles of rich and poor.
Bright working-class youngsters mix with children of every other level of ability in elementary school, but they are increasingly likely to be drawn away to the more intellectually homogeneous high school courses, thence to college. Much of the cognitive talent that used to be in the working-class neighborhood is being whisked up and out of the community through an educational system that is increasingly driven by academic performance. Because of residential segregation, the children of lawyers, physicians, college professors, engineers, and business executives tend to go to schools with each other’s children, and seldom with the children of cab drivers or assembly-line workers, let alone with the children of welfare recipients or the chronically unemployed. They may never go to school with children representative of the whole range of cognitive ability. This tendency is exacerbated by another force working in the background, genes.
Twenty years ago, one of us wrote a book that created a stir because it discussed the heritability of IQ and the relationship of intelligence to success in life, and foresaw a future in which socioeconomic status would increasingly be inherited. The logic of the argument was couched in a syllogism:
If differences in mental abilities are inherited, and
If success requires those abilities, and
If earnings and prestige depend on success,
Then social standing (which reflects earnings and prestige) will be based to some extent on inherited differences among people.24
As stated, the syllogism is not fearsome. If intelligence is only trivially a matter of genes and if success in life is only trivially a matter of intelligence, then success may be only trivially inherited.
How Much Is IQ a Matter Genes?
In fact, IQ is substantially heritable. The state of knowledge does not permit a precise estimate, but half a century of work, now amounting to hundreds of empirical and theoretical studies, permits a broad conclusion that the genetic component of IQ is unlikely to be smaller than 40 percent or higher than 80 percent.25 The most unambiguous direct estimates, based on identical twins raised apart, produce some of the highest estimates of heritability.26 For purposes of this discussion, we will adopt a middling estimate of 60 percent heritability, which, by extension, means that IQ is about 40 percent a matter of environment. The balance of the evidence suggests that 60 percent may err on the low side.
Because IQ and genes has been such a sensitive topic, it is worth a short digression to give some idea of where these estimates come from and how trustworthy they are.
First, consider the question that heads this section, not its answer. What we want to know is how much of the variation in IQ in a population—the aggregated differences among the individuals27—is due to variations in genetic endowments and how much is due to variations in environment. If all the population variation in IQ is due to variations in environment, then the heritability is 0;28 if half is due to environmental variations, it is .5; if none is due to environmental variations, it is 1.0. Heritability, in other words, is a ratio that ranges between 0 and 1 and measures the relative contribution of genes to the variation observed in a trait.29
Specialists have come up with dozens of procedures for estimating heritability. Nonspecialists need not concern themselves with nuts and bolts, but they may need to be reassured on a few basic points. First, the heritability of any trait can be estimated as long as its variation in a population can be measured. IQ meets that criterion handily. There are, in fact, no other human traits—physical or psychological—that provide as many good data for the estimation of heritability as the IQ. Second, heritability describes something about a population of people, not an individual. It makes no more sense to talk about the heritability of an individual’s IQ than it does to talk about his birthrate. A given individual’s IQ may have been greatly affected by his special circumstances even though IQ is substantially heritable in the population as a whole. Third, the heritability of a trait may change when the conditions producing variation change. If, one hundred years ago, the variations in exposure to education were greater than they are now (as is no doubt the case), and if education is one source of variation in IQ, then, other things equal, the heritability of IQ was lower then than it is now.
This last point is especially important in the modern societies, with their intense efforts to equalize opportunity. As a general rule, as environments become more uniform, heritability rises. When heritability rises, children resemble their parents more, and siblings increasingly resemble each other; in general, family members become more similar to each other and more different from people in other families. It is the central irony of egalitarianism: Uniformity in society makes the members of families more similar to each other and members of different families more different.
Now for the answer to the question, How much is IQ a matter of genes? Heritability is estimated from data on people with varying amounts of genetic overlap and varying amounts of shared environment. Broadly speaking, the estimates may be characterized as direct or indirect.30 Direct estimates are based on samples of blood relatives who were raised apart. Their genetic overlap can be estimated from basic genetic considerations. The direct methods assume that the correlations between them are due to the shared genes rather than shared environments because they do not, in fact, share environments, an assumption that is more or less plausible, given the particular conditions of the study. The purest of the direct comparisons is based on identical (monozygotic, MZ) twins reared apart, often not knowing of each other’s existence. Identical twins share all their genes, and if they have been raised apart since birth, then the only environment they shared was that in the womb. Except for the effects on their IQs of the shared uterine environment, their IQ correlation directly estimates heritability. The most modern study of identical twins reared in separate homes suggests a heritability for general intelligence between .75 and .80, a value near the top of the range found in the contemporary technical literature.31 Other direct estimates use data on ordinary siblings who were raised apart or on parents and their adopted-away children. Usually, the heritability estimates from such data are lower but rarely below .4.32
Indirect methods compare the IQ correlations between people with different levels of shared genes growing up in comparable environments—siblings versus half-siblings or versus cousins, for example, or MZ twins versus fraternal (dizygotic, DZ) twins, or nonadoptive siblings versus adoptive siblings. The underlying idea is that, for example, if full siblings raised in the same home and half-siblings raised in the same home differ in their IQ correlations, it is because they differ in the proportion of genes they share: full siblings share about 50 percent of genes, half siblings about 25 percent. Similarly, if siblings raised in unshared environments and cousins raised in unshared environments differ in their IQ correlations, it is because of the differing degrees of genetic overlap between cousins and siblings and not because of differing environmental influences, which are unshared by definition. And so on. Fleshed out in some sort of statistical model, this idea makes it possible to estimate the heritability, but the modeling can get complex. Some studies use mixtures of direct and indirect methods.33
The technical literature is filled with varying estimates of the heritability of IQ, owing to the varying models being used for estimation and to the varying sets of data. Some people seem eager to throw up their hands and declare, “No one knows (or can know) how heritable IQ is.” But that reaction is as unwarranted as it is hasty, if one is content, as we are, to accept a range of uncertainty about the heritability that specialists may find nerve-racking. We are content, in other words, to say that the heritability of IQ falls somewhere within a broad range and that, for purposes of our discussion, a value of .6 ±.2 does no violence to any of the competent and responsible recent estimates. The range of .4 to .8 includes virtually all recent (since 1980) estimates—competent, responsible, or otherwise.34
Recent studies have uncovered other salient facts about the way IQ scores depend on genes. They have found, for example, that the more general the measure of intelligence—the closer it is to g—the higher is the heritability.35Also, the evidence seems to say that the heritability of IQ rises as one ages, all the way from early childhood to late adulthood.36 This means that the variation in IQ among, say, youths ages 18 to 22 is less dependent on genes than that among people ages 40 to 44. Most of the traditional estimates of heritability have been based on youngsters, which means that they are likely to underestimate the role of genes later in life.
Finally, and most surprisingly, the evidence is growing that whatever variation is left over for the environment to explain (i.e., 40 percent of the total variation, if the heritability of IQ is taken to be .6), relatively little can be traced to the shared environments created by families.38 It is, rather, a set of environmental influences, mostly unknown at present, that are experienced by individuals as individuals. The fact that family members resemble each other in intelligence in adulthood as much as they do is very largely explained by the genes they share rather than the family environment they shared as children. These findings suggest deep roots indeed for the cognitive stratification of society.
The Syllogism in Practic
The heritability of IQ is substantial. In Chapters 2 and 3, we presented evidence that the relationship of cognitive ability to success in life is far from trivial. Inasmuch as the syllogism’s premises cannot be dismissed out of hand, neither can its conclusion that success in life will be based to some extent on inherited differences among people.39
Furthermore, a variety of other scientific findings leads us to conclude that the heritability of success is going to increase rather than diminish. Begin with the limits that heritability puts on the ability to manipulate intelligence, by imagining a United States that has magically made good on the contemporary ideal of equality. Every child in this imaginary America experiences exactly the same environmental effects, for good or ill, on his or her intelligence. How much intellectual variation would remain? If the heritability of IQ is .6, the standard deviation of IQ in our magical world of identical environments would be 11.6 instead of 15 (see the note for how this calculation is done)—smaller, but still leaving a great deal of variation in intellectual talent that could not be reduced further by mere equalization.40 As we noted earlier, when a society makes good on the ideal of letting every youngster have equal access to the things that allow latent cognitive ability to develop, it is in effect driving the environmental component of IQ variation closer and closer to nil.
The United States is still very far from this state of affairs at the extremes. If one thinks of babies growing up in slums with crack-addicted mothers, at one extreme, compared to children growing up in affluent, culturally rich homes with parents dedicated to squeezing every last IQ point out of them, then even a heritability of .6 leaves room for considerable change if the changes in environment are commensurably large. We take up the evidence on that issue in detail in Chapter 17, when we consider the many educational and social interventions that have attempted to raise IQ. But those are, by definition, the extremes, the two tails of the distribution of environments. Moving a child from an environment that is the very worst to the very best may make a big difference. In reality, what most interventions accomplish is to move children from awful environments to ones that are merely below average, and such changes are limited in their potential consequences when heritability so constrains the limits of environmental effects.41
So while we can look forward to a future in which science discovers how to foster intelligence environmentally and how to use the science humanely, inherited cognitive ability is now extremely important. In this sense, luck continues to matter in life’s outcomes, but now it is more a matter of the IQ handed out in life’s lottery than anything else about circumstances. High cognitive ability as of the 1990s means, more than even before, that the chances of success in life are good and getting better all the time, and these are decreasingly affected by , the social environment, which by extension indicates that they must be increasingly affected by genes. Holding these thoughts in mind, now consider the phenomenon known as assortative mating.
Love, Marriage, and IQ
The old saw notwithstanding, opposites do not really attract when it comes to love and marriage. Likes attract. In one of the classic papers, originally published in 1943, two sociologists studied 1,000 engaged couples in Chicago, expecting to find at least some traits in which opposites did indeed attract. But out of fifty-one social characteristics studied, the sign of the correlation was positive for every single one. For all but six of the fifty-one traits, the correlations were statistically significant.42 Modest but consistently positive correlations have been found for a wide variety of physical traits as well, ranging from stature (the correlations from many studies average about +.25) to eye color (also averaging about +.25, even within national populations).43
Of the many correlations involving husbands and wives, one of the highest is for IQ. In most of the major studies, the correlation of husband and wife IQ has been in the region of .4, though estimates as low as .2 and as high as .6 have been observed. Jensen’s review of the literature in the late 1970s found that the average correlation of forty-three spouse correlations for various tests of cognitive ability was +.45, almost as high as the typical correlation of IQs among siblings.44
If the Propensity to Mate by Cognitive Ability Has Remained the Same:
When the propensity to mate by cognitive ability is combined with the educational and occupational stratification we have described, the impact on the next generation will be larger than on the previous one, even if the underlying propensity to mate by cognitive ability remains the same.
Consider 100 Harvard/Radcliffe marriages from the class of 1930 versus another 100 from the class of 1964. We stipulate that the propensity to marry people of similar intelligence has not changed in the intervening thirty-four years. Nonetheless, the ones who marry in 1964 will produce a set of children with considerably higher mean IQ than the ones who married in 1930, because the level of intelligence at Harvard and Radcliffe had risen so dramatically.
How much difference can it make? If the average Harvard man in the class of 1930 married the average Radcliffe woman in the same graduating class—as far as we can tell, both would have had IQs of about 117—then the expected mean IQ of their children, after taking regression to the mean into account, will be about 114, or at the 82d percentile.45 But average Harvard and Radcliffe newlyweds in the class of 1964 were likely to have children with a mean IQ of about 124, at the 95th percentile. In terms of distributions rather than averages, about a third of the children of the Harvard newlyweds of 1930 could be expected to have IQs of less than 110—not even college material by some definitions.46 In contrast, only 6 percent of the children of the Harvard newlyweds of 1965 could be expected to fall below this cutoff. Meanwhile, only about 22 percent of the children of the 1930 newlyweds could be expected to match or exceed the average of the children of the 1965 newlyweds. In such numbers lurk large social effects.
If the Propensity to Mate by Cognitive Ability Has Increased:
We have been assuming that the propensity to mate by IQ has remained the same. In reality, it has almost certainly increased and will continue to increase.
We hedge with “almost” because no quantitative studies tell whether assortative mating by intelligence has been increasing recently. But we do know from sociologist Robert Mare of the University of Wisconsin that assortative mating by educational level increased over the period from 1940 to 1987—an increase in “homogamy,” in the sociologists’ language. The increase in homogamy was most pronounced among college-educated persons. Specifically, the odds of a college graduate’s marrying someone who was not a college graduate declined from 44 percent in 1940 to 35 percent in Mare’s most recent data (for 1985 to 1987). The proportion hit a low of 33 percent in the 1980 data.47 Because educational attainment and IQ are so closely linked and became more closely linked in the postwar period, Mare’s results suggest a substantial increase in assortative mating by IQ, with the greatest change occurring at the upper levels of IQ.
Mare identifies some of the reasons for increased homogamy in the trends involving educational attainment, age at leaving school, and age at marriage. But there are a variety of other potential explanations (some of which he notes) that involve cognitive ability specifically. For example, a smart wife in the 1990s has a much greater dollar payoff for a man than she did fifty years ago.48 The feminist movement has also increased the likelihood of marrying by cognitive ability.
First, the feminist revolution in practice (which began in the 1950s, antedating the revolution in rhetoric) drastically increased the odds that bright young women will be thrown in contact with bright young men during the years when people choose spouses. This is most obvious in college, where the proportion of women continuing to college surged from about half the proportion of men in 1950 to equality in 1975.49 It was not just the numbers, however. All of the elite men’s colleges became coeducational, as did many of elite women’s colleges. Strict parietal rules gave way to coeducational dorms. Intelligence has always been an important factor for sorting among prospective mates, but comparison shopping at single-sex colleges like Vassar or Yale was a struggle; the feminist revolution in the universities led to an explosion of information, as it were, that made it easier for the brightest to pair up.
The same phenomenon extended to the workplace. Large proportions of the cognitive elite delay marriage until the later twenties or even thirties. Only a few decades ago, delay tended to dilute the chances of assortative mating by IQ. In a world where the brightest women were usually not in the work force or were in a few restricted occupations, the pool from which a man in his late twenties found a bride were moderated primarily by socioeconomic status; he found his mate among the women he encountered in his neighborhood, church, social organizations, and other settings that were matched mostly by socioeconomic status. But today background status is less important than intelligence. The young man newly graduated from his elite law school joins his elite New York firm, thereupon encountering young women, just as highly selected for cognitive ability as he was, in the adjacent offices at his own firm, at business lunches, across the table in negotiations, on a daily basis. The opportunities for propinquity to work its magic were increased in the workplace too, and will continue to increase in the years to come.
The second effect of feminism is less ponderable but may be important anyway. Not so many years ago, the cliché was true: brains were not considered sexy in a woman, and many men undervalued brains as an asset in a prospective spouse or even felt threatened by smart women. Such attitudes may linger in some men, but feminism has surely weakened them and, to some degree, freed relationships among men and women so that a woman’s potential for occupational success can take as dominant a place in the man’s marriage calculus as it has traditionally taken in the woman’s.50 We speculate that the effect has been most liberating among the brightest. If we are right, then the trends in educational homogamy that Mare has demonstrated are an understated reflection of what is really going on. Intermarriage among people in the top few percentiles of intelligence may be increasing far more rapidly than suspected.
THE LIMITS OF CHURNING
American society has historically been full of churning, as new groups came to this country, worked their way up, and joined the ranks of the rich and powerful. Meanwhile, some of the children of the rich and powerful, or their grandchildren, were descending the ladder. This process has made for a vibrant, self-renewing society. In depressing contrast, we have been envisioning a society that becomes increasingly quiescent at the top, as a cognitive elite moves toward the upper income brackets and runs most of the institutions of society, taking on some of the characteristics of a caste.
Is the situation really so extreme? To some extent, not yet. For example, national surveys still indicate that fewer than 60 percent in the top quartile of intelligence actually complete a bachelor’s degree.51 This would seem to leave a lot of room for churning. But when we focus instead on the students in the top few centiles of cognitive ability (from which the nation’s elite colleges pick almost exclusively), an extremely high proportion are already being swept into the comfortable precincts of the cognitive elite.52 In the NLSY, for example, 81 percent of those in the top 5 percent of IQ had obtained at least a bachelor’s degree by 1990, when the youngest members of the sample were 25 years old.53
When we examine the remaining 19 percent who had not obtained college degrees, the efficiency of American society in pushing the most talented to the top looks even more impressive. For example, only a small portion of that 19 percent were smart students who had been raised in a low-income family and did not get to college for lack of opportunity. Only 6 percent of persons in the top five IQ centiles did not have a college degree and came from families in the lower half of socioeconomic status.54
If this 19 percent of high-IQ persons-without-B.A.s does not fit the stereotype of the deprived student, who were they? Some were becoming members of the cognitive elite even though they do not have a college degree. Bill Gates, college dropout and founder of Microsoft, is the larger-than-life prototype. Five percentage points of the 19 percent were working in one of the high-IQ occupations, indicating that they were probably of the minor-league Bill Gates variety (corroborated by their incomes, which were high). Of the remaining 14 percent who were not working in high-IQ occupations, a quarter had family incomes in excess of $50,000 while they were still only in their late twenties and early thirties, putting them in the top 20 percent of family incomes for their age group.55 In total, roughly half of these smart non-college graduates are already taking their place among the smart college graduates, by virtue of their incomes, their occupations, or both. It seems a safe bet that the neighborhoods where they live and the way they socialize their children are going to be indistinguishable from those of most of their counterparts in the top five centiles who completed college.
There is doubtless some relatively small fraction of those in the top 5 percent intellectually who will never rise to successful positions, whether because of lack of motivation or objective barriers. But what a small percentage of the highly talented they are. And we may add a reminder that we are watching an ongoing process. Think back to Chapter 1 and imagine the trend line from 1900 to 1990 stretched out to, say, 2020. Whatever the number of the cognitive elite who slip between the cracks now, it is a much smaller figure than it was in the 1950s, radically smaller than it was in the 1900s, and presumably it will get smaller still in the future.
These observations have several implications. At a practical policy level, the most obvious is that programs to expand opportunity for the disadvantaged are not going to make much difference in getting the most talented youths to college. An extremely high proportion of those who want to go are already going. The broader implication is that the funneling system is already functioning at a high level of efficiency, thereby promoting three interlocking phenomena:
1. The cognitive elite is getting richer, in an era when everybody else is having to struggle to stay even.
2. The cognitive elite is increasingly segregated physically from everyone else, in both the workplace and the neighborhood.
3. The cognitive elite is increasingly likely to intermarry.
These phenomena are driven by forces that do not lend themselves to easy reconfiguration by politicians. As we leave Part I, here is a topic to keep in the back of your mind: What if the cognitive elite were to become not only richer than everyone else, increasingly segregated, and more genetically distinct as time goes on but were also to acquire common political interests? What might those interests be, and how congruent might they be with a free society? How decisively could the cognitive elite affect policy if it were to acquire such a common political interest?
These issues will return in the last chapters in the book. They are postponed for now, because we must first explore the social problems that might help create such a new political coalition.