Social Behavior and the Prevalence of Low Cognitive Ability - The National Context - The Bell Curve: Intelligence and Class Structure in American Life - Richard J. Herrnstein, Charles Murray

The Bell Curve: Intelligence and Class Structure in American Life - Richard J. Herrnstein, Charles Murray (1996)

Part III. The National Context

Chapter 16. Social Behavior and the Prevalence of Low Cognitive Ability

In this chapter, the question is not whether low cognitive ability causes social problems but the prevalence of low cognitive ability among people who have those problems. It is an important distinction. Causal relationships are complex and hard to establish definitely. The measure of prevalence is more straightforward. For most of the worst social problems of our time, the people who have the problem are heavily concentrated in the lower portion of the cognitive ability distribution. Any practical solution must therefore be capable of succeeding with such people.

This chapter brings together the social behaviors we covered in Part II from a new vantage point. The earlier chapters showed that low cognitive ability raises the risk of living in conditions or behaving in ways that society hopes to change. Now the question concerns prevalence: To what extent does low cognitive ability describe the people thus afflicted? The distinction is more familiar in the medical context. High cholesterol may be a risk factor for heart disease, but most people with heart disease may or may not have high cholesterol. If most people who have heart attacks do not have high cholesterol, then lowering the cholesterol of those with high levels will not do much to reduce the frequency of heart attacks in the population at large. Similarly, to the extent that low cognitive ability is prevalent among people who have the problems we hope to solve, policies that are effective for people with low scores should be sought.

The entire NLSY sample, including the Asian-Americans, American Indians, and other ethnic groups that have hitherto been excluded, are used here. The proportions presented in this chapter are representative of America’s national population for an age cohort that was 26 to 33 as of 1990.

POVERTY

In 1989, the official national statistics revealed that 11.1 percent of persons ages 25 to 34 years old were poor in that year, virtually identical with the 10.9 percent below the poverty line in the NLSY sample ages 25 to 33. So while the NLSY cannot give us a precise figure for overall national poverty, there is no reason to think that the results from it are misleading for young adults. This is in preface to the sobering figure that follows.

Forty-eight percent of the poor in 1989 came from the bottom 20 percent in intelligence

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This graph uses conventions that we follow throughout the chapter: The headline gives the percentage of the population in question (in this instance, the poor) in the bottom 20 percent of IQ, and the scale is the same for each graph. The bars show the percentage of the poor population who come from each decile, marked by the scale on the left. If cognitive ability were irrelevant to poverty, the bars would be of equal height, each at just 10 percent. Adding up the percentages in each bar from left to right gives the cumulative percentage, shown by the black line and the right-hand scale. For example, the first two deciles add up to 48 percent; therefore the black line crosses the 48 percent mark at the second bar. The cumulative scale is a way of showing what proportion of poor people fall below any given decile. For example, in the case of poverty, almost 80 percent of poor people are in or below the fourth decile. If cognitive ability were irrelevant, the line would be a straight diagonal from lower left to the upper right.

In terms of IQ points, the cognitive ability deciles in the figure above, as in all the others in the chapter, correspond to the scores in the table below. The bottom two deciles cut off IQ 87 and below and the top two deciles cut off IQ 113 and above. It may also be useful to recall that most college graduates and almost everyone with a professional degree fall in the ninth and tenth deciles.

IQ Equivalents for the Deciles

Decile

Range

Median

1st

Under 81

74

2d

81-87

84

3d

87-92

90

4th

92-96

94

5th

96-100

98

6th

100-104

102

7th

104-108

106

8th

108-113

110

9th

113-119

116

10th

Above 119

126

The figure tells us forcefully that poverty is concentrated among those with low cognitive ability. The mean IQ of people below the poverty line was 88. A third of them came from the very bottom decile; they had IQs under 81. Eighty-two percent had below-average IQs.

HIGH SCHOOL DROPOUTS

It will come as no surprise to find that most high school dropouts have low intelligence. The figure below shows the results for persons who dropped out of school and did not subsequently obtain a GED. Overall, 94 percent of those who permanently dropped out of school were below average in IQ. As we noted in Chapter 6, this disproportion is not materially affected by analyses limited to persons who took the intelligence test before they dropped out, so it cannot be explained by the effects of a lack of schooling on their IQs.

Two-thirds of high school dropouts came from the bottom 20 percent in intelligence

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Those who drop out of school and later return to get their GED are markedly below the mean of those who finish high school in the normal way, but they are not as severely skewed toward the bottom end of the distribution. Twenty-five percent are in the bottom two IQ deciles, and 69 percent are in the bottom half of the distribution.

MEN AND WORK

The Employed

Year-round employment has only a minor association with cognitive ability. The figure below, based on men who worked fifty-two weeks in 1989, makes this point plainly. We italicize it because, although it is consistent with the analysis presented for whites in Chapter 7, we want to emphasize that the same result applies across ethnic groups.

Seventeen percent of the men who worked year round in 1989 were in the bottom 20 percent of intelligence

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By and large, men who were employed throughout 1982 were spread across the full range of IQs, with only a minor elevation for those in the top four deciles. The mean IQ of year-round workers was 102. Those with low IQ have a statistically tougher time in many ways, but they contribute very nearly their full share to the population of men employed year round, an important fact to remember as a counterweight to most of the other findings in this chapter.

Nonworkers

The prototypical member of the underclass in the public imagination is a young male hanging out on the streets, never working. This amounted to very few men. Only 2.2 percent of NLSY men not in school and not prevented from working because of health problems failed to work at least a week in 1989. But among these 2.2 percent, low cognitive ability predominated. The figure below, limited to civilian men out of school and not physically prevented from working, combines those who said they were unemployed and those who said they had dropped out of the labor force; their common denominator is that they reported zero weeks of working for 1989. The mean IQ of men who did not work at all was 84. Fifty percent were in the bottom decile. Eighty-four percent were below average.

Sixty-four percent of able-bodied men who did not work in 1989 were in the bottom 20 percent of intelligence

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Unemployment

Now we turn to the men not represented in either of the two figures above: men who worked at least some time during 1989 but were out of work for more than four weeks. There was somewhat more unemployment among the lower deciles of IQ, as the figure below shows, but, as the almost straight diagonal line shows, the relationship was not strong. For these men, the mean IQ was 97, three points below average. If we were to add another graph, for men who were out of work for six months but not the full year, it would show a stronger relationship, about halfway between the graph just above and the earlier graph for men who were out of the labor force all year. The general principle is that the longer the period of unemployment, the more prevalent is low IQ. Short-term unemployment is not conspicuously characterized by low IQ; long-term unemployment is.

Twenty-nine percent of able-bodied men who were temporarily out of work in 1989 were in the bottom 20 percent of intelligence

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MEN AND CRIME

The next figure contains the breakdown of the IQs of men in the NLSY who were interviewed in a correctional facility, showing that they had committed at least one offense serious enough to get them locked up. The mean IQ of men who were ever interviewed in a correctional facility was 84. Forty-five percent were concentrated in the bottom decile of cognitive ability. Ninety-three percent of the men were somewhere in the bottom half of the cognitive ability distribution. This high prevalence of low IQ among offenders is consistent with other estimates in the literature, as summarized in Chapter 11.

Sixty-two percent of men ever interviewed in jail or prison came from the bottom 20 percent of intelligence

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WOMEN AND WELFARE

We start with women who have ever received welfare. The data are shown in the figure below. Overall, the mean IQ of women who ever received welfare was 89. About 85 percent of them were below average in IQ, and fewer than 4 percent had IQs in the top two deciles.

For chronic welfare recipients, defined as women who had received welfare for at least five years by 1990, the cognitive distribution was even lower.1 As the figure shows , 57 percent of chronic welfare mothers were in the bottom two deciles of IQ, 88 percent were in the bottom half of the distribution, and their mean IQ was 86. Just as low IQ was increasingly prevalent as the level of male unemployment increased, so also is low IQ more prevalent among mothers as their dependency on welfare rises.

Forty-five percent of women who ever received welfare are in the bottom 20 percent of intelligence

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Fifty-seven percent of chronic welfare recipients are in the bottom 20 percent of intelligence

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OUTCOMES FOR CHILDREN

In this section, we describe the prevalence of low IQ among the mothers of children with various problems. That is, we are presenting an answer to the question, “If I am trying to deal with a certain problem regarding the children of young adults, what can I assume about the intelligence of their mothers?”

We begin with the overriding fact that, as of 1990, the NLSY mothers as a group were markedly below average in IQ. Their mean IQ was 95.7. Fourteen percent of NLSY children were born to mothers in the bottom decile of IQ; 27 percent: to mothers in the bottom two deciles; 62 percent to mothers in the bottom half of the distribution. Thus, for example, a problem involving NLSY children will “ordinarily” show that 62 percent of the children have mothers with below-average IQ. As will be clear, the observed proportions of low-IQ mothers are often considerably elevated above that expectation.2 But these benchmark figures must be kept in mind when interpreting all the analyses involving children.

Illegitimacy

We start with the children who are born to unmarried women (see the figure below). The mean IQ of mothers of children born out of wedlock was 87.3 Of all illegitimate children in the NLSY sample, almost one out of three was born to a mother in the bottom 10 percent of the intelligence distribution, with an IQ under 81, and 85 percent were born to women in the bottom half of the cognitive ability distribution.

Fifty-two percent of illegitimate children were born to mothers in the bottom 20 percent of intelligence

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Restricting the analysis to those children who are most at risk, these percentages, already extreme, become even more bunched at the lower end of the distribution. Consider children who fit the archetype of the child at risk: born to a poor, single, teenage girl (with poverty measured in the year prior to giving birth). Almost two out of three (64 percent) of such children were born to women in the bottom 20 percent of the cognitive ability distribution. Ninety-five percent of them were born to women in the bottom half.

Other Forms of Single Parenthood

The figure below shows the proportion of NLSY children born to a married couple but living (in 1990) with just their mothers because of divorce or separation. First, a caution: The profile we are about to present may change in the future because so many of the expected divorces among the NLSY sample have not yet occurred. For women who had ever been married in the 25 to 33 age range as of 1990, we may, however, ask: Among their children who were living in mother-only families as of 1990, what is the distribution of the mother’s intelligence?

Thirty-one percent of children living with divorced or separated mothers had mothers with IQs in the bottom 20 percent of intelligence

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Divorced and separated mothers averaged an IQ of 93.4 More than half of all children living with their divorced or separated mothers in the NLSY were born to women in the bottom 30 percent of the IQ distribution. Seventy-six percent were born to women in the bottom half of the distribution. Remember that there is no confounding with illegitimacy; all children born out of wedlock are excluded from this sample. The prevailing notion that separation and divorce are so endemic that they affect everyone more or less equally is wrong as regards cognitive ability, at least in this age group.

Perhaps the differences will even out to some extent in the long run. Brighter women get married and have their children later. In the NLSY sample, their marriages have had less time to break up than those for women lower in the distribution. Only time will tell whether and how much the distribution in the graph above will change in the years to come. At this point, the skew is notable and clear.

Pulling together the data on illegitimacy and other forms of single parenthood, here are a few key points:

· Within the bottom two deciles of intelligence, illegitimacy is more common than divorce or separation as the source of single parenthood.

· Beginning with the third decile, divorce and separation become an equal or predominant source of single parenthood.

· The bottom half of the cognitive ability distribution accounts for 82 percent of all children in single-parent homes (combining illegitimacy with divorce or separation) as of 1990.

Low-Birth-Weight Babies

Among whites, the chances of having a low-birth-weight baby were associated with IQ, not socioeconomic background, when both variables were taken into account (Chapter 10). The prevalence of low-birth-weight babies among women in the bottom half of the distribution persists when the entire NLSY sample is considered (the figure below). Mothers with low-birth-weight babies averaged an IQ of 89. Almost three out of four (74 percent) mothers were in the bottom half of the IQ distribution.

Forty-five percent of low-birth-weight babies had mothers in the bottom 20 percent of intelligence

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Deprived Home Environments

Chapter 10 discussed the HOME inventory, a measure combining many indicators of both emotional support (for example, disciplinary style) and cognitive stimulation (for example, reading to the child). Here, we examine children whose HOME scores put them in the bottom 10 percent of environments (using national norms for the HOME inventory).

The mean IQ of mothers of children in the worst home environments was 86. Three out of eight had IQs below 81; 86 percent had IQs below 100. The figure below combines the results for children in all age groups. There were some age differences, however: Generally, the concentration of the worst environments among mothers with low cognitive ability got worse as the children got older. For children ages 3 to 5 who were in the worst home environments, 59 percent had mothers with IQs in the bottom two deciles. For children 6 and older, the figure was 65 percent.

Fifty-six percent of all children from bottom decile in home environment were born to mothers in the bottom 20 percent of intelligence

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Children in Poverty

The proportion of children living in poverty is one of the most frequently cited statistics in public policy debates and one of the most powerful appeals to action. In considering what actions might be taken, and what will and won’t work, keep the following figure in mind. It shows the distribution of maternal cognitive ability among children who spent their first three years below the poverty line. Mothers whose children lived in poverty throughout their first three years averaged an IQ of 84. Forty-one percent had mothers in the very bottom decile in cognitive ability. In all, 93 percent were born to women in the bottom half of the IQ distribution. Of all the social problems examined in this chapter, poverty among children is preeminently a problem associated with low IQ—in this case, low IQ among the mothers.

Developmental Problems Among Children

The prevalence of developmental problems among children is skewed toward the lower half of the IQ distribution. Rather than present graphs for each of them, the table below summarizes a consistent situation. See Chapter 10 for a description of the indexes« Low IQ is prevalent among the mothers of children with each of these developmental problems, but none shows as strong a concentration as the developmental indicator we consider the most important for eventual social adjustment: the child’s own IQ. The figure below is limited to the cognitive ability of children ages 6 and older when they took the test.

Sixty-three percent of children who lived in poverty throughout the first three years had mothers in the bottom 20 percent of intelligence

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Prevalence of Low IQ Among Mothers of Children with Developmental Problems

Children in the Worst Decile on:

Percentage of These Children with Mothers in Bottom:

Mean IQ of Mothers

20% of IQ

50% of IQ

Friendliness index, 12-23 mos.

49

82

88

Difficulty index, 12-23 mos.

40

71

91

Motor and social development index, birth-47 mos.

38

67

93

Behavioral problems index, children ages 4-11 yrs.

42

78

90

Seventy-two percent of children in the bottom decile of IQ had mothers in the bottom 20 percent of intelligence

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The mean IQ of mothers of children who scored in the bottom decile of a childhood intelligence test was 81.5 Overall, 94 percent of these children had mothers with IQs under 100. The extreme concentration of low IQ among the children of low-IQ mothers is no surprise. That it is predictable does not make the future any brighter for these children.

CONCLUDING REMARKS

Let us conclude on a brighter note, after so unrelenting a tally of problems. You will recall from Chapter 12 that we developed a Middle Class Values Index. To qualify for a score of “yes,” an NLSY person had to be married to his or her first spouse, in the labor force (if a man), bearing children within wedlock (if a woman), and never have been interviewed in jail How did the NLSY sample break down by IQ? The results are set out in the figure.

Ten percent of people scoring “yes” on the Middle Class Values Index were in the bottom 20 percent of intelligence

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The mean IQ of those who scored “yes” was 104« Those in the bottom two deciles contributed only about 10 percent, half of their proportional share. Those in the bottom half of the cognitive distribution contributed 37 percent. As in the case of year-round employment, the skew toward those in the upper half of the cognitive ability distribution is not extreme. This reminds us again more generally that most people in the lower half of the cognitive distribution are employed, out of poverty, not on welfare, married when they have their babies, providing a nurturing environment for their children, and obeying the law.

We must add another reminder, however. There is a natural tendency to review these figures and conclude that we are really looking at the consequences of social and economic disadvantage, not intelligence. But in Part II, we showed that for virtually all of the indicators reviewed in this chapter, controlling for socioeconomic status does not get rid of the independent impact of IQ. On the contrary, controlling for IQ often gets rid of the independent impact of socioeconomic status. We have not tried to present the replications of those analyses for all ethnic groups combined, but they tell the same story.

The lesson of this chapter is that large proportions of the people who exhibit the behaviors and problems that dominate the nation’s social policy agenda have limited cognitive ability. Often they are near the definition for mental retardation (though the NLSY sample screened out people who fit the clinical definition of retarded). When the nation seeks to lower unemployment or lower the crime rate or induce welfare mothers to get jobs, the solutions must be judged by their effectiveness with the people most likely to exhibit the problem: the least intelligent people. And with that, we reach the practical questions of policy that will occupy us for the rest of the book.