The early studies from the United States, England, France, and Greece all seemed to confirm the reality of dysgenesis.15 In the 1930s, the eminent psychometrician Raymond Cattell was predicting a loss of 1.0 to 1.5 IQ points per decade,16 while others were publishing estimated losses of 2 to 4 IQ points per generation.17 In 1951, another scholar gloomily predicted that “if this trend continues for less than a century, England and America will be well on the way to becoming nations of near half-wits.”18 The main source of their pessimism was that the average IQ in large families was lower than in smaller families.
Then came a period of optimism. Its harbinger was Frederick Osborn’s Eugenic Hypothesis, first stated in 1940, which foresaw a eugenic effect arising from greater equality of social and economic goods and wider availability of birth control.19 In the late 1940s, data began to come in that seemed to confirm this more sanguine view. Surveys in Scotland found that Scottish school children were getting higher IQs, not lower ones, despite the familiar negative relationship between family size and IQ.20 Examining this and other new studies, Cattell reconsidered his position, concluding that past estimates might not have adequately investigated the relationship between intelligence and marriage rates, which could have skewed their results.21
The new optimism got a boost in 1962 with the publication of “Intelligence and Family Size: A Paradox Resolved,” in which the authors, using a large Minnesota sample, showed how it was possible to have both a negative relationship between IQ and family size and, at the same time, to find no dysgenic pattern for IQ.22 The people who had no children, and whose fertilities were thus omitted from the earlier statistics, the authors suggested, came disproportionately from the lower IQ portion of the population. From the early 1960s through 1980, a series of studies were published showing the same radically changed picture: slowly rising or almost stable intelligence from generation to generation, despite the lower average IQs in the larger families.23
The optimism proved to be ephemeral. As scholars examined new data and reexamined the original analyses, they found that the optimistic results turned on factors that were ill understood or ignored at the time the studies were published. First, comparisons between successive generations tested with the same instrument (as in the Scottish studies) were contaminated by the Flynn effect, whereby IQ scores (though not necessarily cognitive ability itself) rise secularly over time (see Chapter 13). Second, the samples used in the most-cited optimistic studies published in the 1960s and 1970s were unrepresentative of the national population. Most of them came from nearly all-white populations of states in the upper Midwest.24 Two of the important studies published during this period were difficult to interpret because they were based not only on whites but on males (estimating fertility among males poses numerous problems, and male fertility can be quite different from female fertility) and on samples that were restricted to the upper half of the ability distribution, thereby missing what was going on in the lower half.25
Apart from these technical problems, however, another feature of the studies yielding optimistic results in the 1960s and 1970s limited their applicability: They were based on the parents of the baby boomers, the children born between 1945 and about 1960. In 1982, demographer Daniel Vining, Jr., opened a new phase of the debate with the publication of his cautiously titled article, “On the Possibility of the Reemergence of a Dysgenic Trend with Respect to Intelligence in American Fertility Differentials.”26 Vining presented data from the National Longitudinal Survey cohorts selected in 1966 and 1968 (the predecessors of the much larger 1979 NLSY sample that we have used so extensively) supporting his hypothesis that people with higher intelligence tend to have fertility rates as high as or higher than anyone else’s in periods of rising fertility but that in periods of falling birth rates, they tend to have lower fertility rates. The American fertility rate had been falling without a break since the late 1950s, as the baby boom subsided, and Vining suspected that dysgenesis was again underway.
Then two researchers from the University of Texas, Marian Van Court and Frank Bean, finding no evidence for any respite during the baby boom in a nationally representative sample, determined that the childless members of the sample were not disproportionately low IQ at all; on the contrary, they had slightly higher IQs than people with children. Van Court and Bean concluded that the United States had been experiencing an unbroken dysgenic effect since the early years of the century.27
Since then, all the news has been bad. Another study of the upper Midwest looked at the fertilities in the mid-1980s of a nearly all-white sample of people in Wisconsin who had been high school seniors as of 1957 and found a dysgenic effect corresponding to about 0.8 IQ point per generation.28 A 1991 study based on a wholly different approach and using the NLSY suggests that 0.8 per generation may be an underestimate.29 This study estimated the shifting ethnic makeup of the population, given the differing intrinsic birth rates of the various ethnic groups. Since the main ethnic groups differ in average IQ, a shift in America’s ethnic makeup implies a change in the overall average IQ. Even disregarding the impact of differential fertility within ethnic groups, the shifting ethnic makeup by itself would lower the average American IQ by 0.8 point per generation. Since the differential fertility within those ethnic groups is lowering the average score for each group itself (as we show later in the chapter), the 0.8 estimate is a lower bound of the overall population change.
To summarize, there is still uncertainty about whether the United States experienced a brief eugenic interlude after World War II. Van Court and Bean conclude it has been all downhill since the early part of the twentieth century; other researchers are unsure.30 There is also uncertainty deriving from the Flynn effect. James Flynn has by now convinced everyone that IQ scores rise over time, more or less everywhere they are studied, but there remains little agreement about what that means. For those who believe that the increase in scores represents authentic gains in cognitive ability, the dysgenic effects may be largely swamped by overall gains in the general environment. For those who believe that the increases in scores are primarily due to increased test sophistication without affecting g, the Flynn effect is merely a statistical complication that must be taken into account whenever comparing IQ scores from different points in time or across different cultures.
But within the scholarly community, there is little doubt about differential fertility or about whether it is exerting downward pressure on cognitive ability. Further, the scholarly debate of the last fifty years has progressed: The margin of error has narrowed. Scientific progress has helped clarify the dysgenic effects without yet producing a precise calibration of exactly how much the distribution of cognitive ability is declining. This leads to our next topic, the current state of affairs.
DYSGENIC PRESSURES IN AMERICA IN THE EARLY 1990S
Foretelling the future about fertility is a hazardous business, and foretelling it in terms of IQ points per generation is more hazardous still. The unknowns are too many. Will the ranks of career women continue to expand? Or might our granddaughters lead a revival of the traditional family? How will the environmental aspects of cognitive development change (judging from what has happened to SAT scores, it could be for worse as well as better)?31 Will the Flynn effect continue? Even if it does, what does it mean? No one has any idea how these countervailing forces might play out.
For all these reasons, we do not put much confidence in any specific predictions about what will happen to IQ scores decades from now. But we can say with considerable confidence what is happening right now, and the news is worrisome.32 There are three major factors to take into account: the number of children born to women at various IQ levels, the age at which they have them, and the cognitive ability of immigrants.
Cognitive Ability and Number of Children
Demographers often take a lifetime fertility of about 2.1 births as the dividing line between having enough children to replenish the parent generation and having too few.33 Bear that in mind w
hile examining the figure below showing the “completed fertility”—all the babies they have ever had—of American women who had virtually completed their childbearing years in 1992, broken down by their educational attainment. Overall, college graduates had 1.56 children, one child less than the average for women without a high school diploma. Let us consider the ratio of the two fertilities as a rough index of the degree to which fertility is tipped one way or the other with regard to education. A ratio greater than 1.0 says the tip is toward the lower educational levels. The actual ratio is 1.71, which can be read as 71 percent more births among high school dropouts than among women who graduated from college. At least since the 1950s, the ratio in the United States has been between 1.5 and 1.85.34
The higher the education, the fewer the babies
Source: Bachu 1993, Table 2.
What does this mean for IQ? We may compute an estimate by using what we know about the mean IQs of the NLSY women who reached various levels of education. Overall, these most recent data on American fertility (based on women ages 35 to 44 in 1992, when the survey was taken) implies that the overall average IQ of American mothers was a little less than 98.35 This is consistent with the analyses of American fertility that suggest a decline of at least 0.8 point per generation.
This estimate is strengthened by using an altogether different slice of the national picture, based on the birth statistics for virtually all babies born in the United States in a given year, using the data compiled in Vital Statistics by the National Center for Health Statistics (NCHS). The most recent data available as we write, for 1991, provide modestly good news: The proportions of children born to better-educated women—and therefore higher-IQ women, on average—have been going up in the last decade. The proportion of babies born to women with sixteen or more years of school (usually indicating a college degree or better) rose from 4.8 percent in 1982 to 5.9 percent in 1991. The proportion of babies born to women with something more than a high school diploma rose from 34-2 percent to 38.2 percent—small changes but in the right direction. The bad news is that the proportion of children born to women with less than a high school education has risen slightly over the last decade, from 22 percent to 24 percent, attributable to an especially steep rise among white women since 1986.
In trying to use the educational information in Vital Statistics to estimate the mean IQ of mothers in 1991, it is essential to anticipate the eventual educational attainment of women who had babies while they were still of school age. After doing so, as described in the note,36 the estimated average IQ of women who gave birth in 1991 was 98. Considering that census data and the Vital Statistics data come from different sources and take two different slices of the picture, the similarities are remarkable. The conclusion in both cases is that differential fertility is exerting downward pressure on IQ. At the end of the chapter, we show how much impact changes of this size may have on American society.
What of evidence about dysgenesis in the NLSY itself? As of 1990, the women of the NLSY, ages 25 to 33, still had many childbearing years ahead. Presumably the new births will be weighted toward more highly educated women with higher IQs. Therefore the current mean IQ of the mothers of the NLSY children will rise. Currently, however, it stands at less than 96.37
Cognitive Ability and Mother’s Age
Population growth depends not just on the total number of children women have but on how old they are when they have them. The effect is dysgenic when a low-IQ group has babies at a younger age than a high-IQ group, even if the total number of children born in each group eventually is the same. Because this conclusion may not be intuitively obvious, think of a simplified example. Suppose that over several generations Group A and Group B average exactly the same number of children, but all the women in Group A always have their babies on their twentieth birthday and all the women in Group B have their children on their thirtieth birthday. The women in group A will produce three generations of children to every two produced by Group B. Something like this has been happening in the United States, as women of lower intelligence have babies younger than women of higher intelligence. The NLSY once again becomes the best source, because it provides age and education along with IQ scores.
The oldest women in the NLSY had reached the age of 33 in 1990, by which time the great majority of first births have taken place.38 We can thus get a good idea of how age at first birth or average age at all births varies with cognitive ability, recognizing that a small minority of women, mostly highly educated and at the upper portion of the IQ distribution, will eventually nudge those results slightly.39 We will not try to compensate for these missing data, because the brunt of our argument is that the timing of births has a dysgenic effect. The biases in the data, reported in the table below for women who were 30 or older, tend to understate the true magnitude of age differences by IQ.40
The average age at first birth was a few months past the 23d birthday. This varied widely, however, by cognitive class. Combining all the ethnic groups in the NLSY, women in the bottom 5 percent of intelligence have their first baby more than seven years younger than women in the top 5 percent. When these figures are computed for the average age for all births (not just the first birth, as in the table), women in the bottom 5 percent have their babies (or all of the ones they have had by their early thirties) at an average of five and a half years earlier. This gap will grow, not shrink, as the NLSY women complete their childbearing years. Even using the current figures, women in the bottom 5 percent of the IQ distribution will have about five generations for every four generations of the top 5 percent. A large and often ignored dysgenic pressure from differences in age at birth is at work.
Age at Childbearing
Cognitive Class Mean Age at First Birth
I Very bright 27.2
II Bright 25.5
III Normal 23.4
IV Dull 21.0
V Very dull 19.8
Overall average 23.1
ETHNIC DIFFERENCES IN FERTILITY
Whatever the ethnic differences in cognitive ability are now, they may change if ethnic groups differ in the extent to which their fertilities are dysgenic or not. In the long run, the vector of demographic trends in intelligence—converging or diverging across ethnic groups—could profoundly affect America’s future.
Fertility Rates by Ethnicity
In the 1992 analysis of American fertility using the Current Population Survey (CPS) to which we referred for a national estimate of dysgenesis, women ages 35 to 44 had given birth to an average of 1.94 children: 1.89 for white women, 2.23 for black women, and 2.47 for Latino women.41 Similar or larger ethnic differences have characterized fertility data for as long as such data have been available, and they have led to a widespread belief that something in black and Latino culture leads them to have larger numbers of children than whites do. We do not dispute that culture can influence family size—the Catholic tradition among Latinos may foster high overall birth rates, for example—but the trends for the three groups are similar once the role of educational level is held constant. Consider the figure below, based on the 1992 CPS study of fertility, again using women in the 35 to 44 age group who have nearly completed their childbearing years.
This figure represents almost total lifetime fertilities, and it tells a simple story. In all three groups of women, more education means lower fertility. The two minority groups have higher overall fertility, but not by much when education is taken into account. Given the known relationship between IQ and educational attainment, fertility is also falling with rising IQ for each ethnic group. Indeed, if one tries to look into this relationship by assigning IQ equivalents based on the relationship of educational attainment and cognitive ability in the NLSY, it appears that after equating for IQ, black women at a given IQ level may have lower fertility rates than either white or Latino women.42
Fertility falls as educational level rises in similar fashion for black, white, and Latino women
Source: Bachu 1993, Table 2
.
May we then conclude that whites, blacks, and Latinos are on a downhill slope together, neither converging nor diverging in IQ? No, for two reasons. The first is that each ethnic group has different proportions of women at different IQ levels. For example, black women with IQs of 90 and below probably have a fertility rate no higher than that of white women with the same IQs. But even so, only 15 percent of white women in the NLSY fall in the 90-and-below range, compared with 52 percent of black women. The relatively higher fertility rates of women with low IQs therefore have a larger impact on the black population as a whole than on the white. Even if two ethnic groups have equal birth rates at a given IQ, one group may have a larger proportion of its babies than the other at that IQ. This is illustrated by the next table, which uses the NLSY to see what the next generation looks like so far, when the women of the NLSY had reached the ages of 25 to 33.
The Next Generation So Far, for Three Ethnic Groups in the NLSY
As of 1990, the Percentage of Children Born to Women with:
IQs Less than 90 IQs Higher than 110
Whites 19 22
Blacks 69 2
Latinos 64 2
National population 33 15
Deciding whether the discrepancy between whites and both blacks and Latinos implies an increasing gap in cognitive ability would require extensive modeling involving many assumptions. On the face of it, the discrepancies are so dramatically large that the probability of further divergence seems substantial. Furthermore, insofar as whites have the highest proportion of college-educated women who are delaying childbirth, the gap between whites and the other minorities is more likely to increase than to diminish as the NLSY women complete their childbearing years.
The Bell Curve: Intelligence and Class Structure in American Life Page 40