The Intelligence Paradox: Why the Intelligent Choice Isn't Always the Smart One - Satoshi Kanazawa 2012
Intelligence and the Number of Children
Why Intelligent People Are the Ultimate Losers in Life
According to one study of the Swedish population, 99.7% of women and 96.5% of men complete their lifetime reproduction by the time they are 45.1 In other words, very few people—women or men—have children after they are 45. By Sweep 7 in 2004—2005, the NCDS respondents were 46—47 years old, so I can safely assume that most of them have completed their reproductive careers by Sweep 7. I will therefore look at how many children they have actually had before Sweep 7.
By the time they are 47, childhood general intelligence has a significantly negative effect on the number of children NCDS respondents have had only among women, not among men. More intelligent women have had fewer children than less intelligent women, but more intelligent men have not had fewer children than less intelligent men. Recall that, in a bivariate analysis, both more intelligent men and more intelligent women wanted to have fewer children when they were 23. It therefore means that more intelligent women have been able to fulfill their desire to have fewer children a quarter of a century later, at the end of their reproductive careers, but more intelligent men have not been able to fulfill their similar desire to have fewer children.
Even net of the same social and demographic characteristics as before, in the analysis of the desired number of children above, more intelligent women have fewer children in their lifetimes than less intelligent women. In contrast, more intelligent men, despite having wanted to have fewer children at age 23, do not actually have fewer children by age 47. Among women, childhood general intelligence significantly decreases the number of children they have had in their lifetimes. Among men, it does not. While the effect of childhood general intelligence on women's fertility is consistent with the prediction of the Intelligence Paradox, the lack of the same effect among men is inconsistent with it.
As you can see in Figures 12.3 and 12.4, more intelligent women are significantly more likely to remain childless—and significantly less likely to become parents—than less intelligent women. The mean childhood IQ of women who have remained childless for life is 105.3, whereas the mean childhood IQ of women who have become parents is 101.7. The difference in mean childhood IQ between the two categories of women is very large and statistically significant.
Figure 12.3 Association between mean childhood IQ and parenthood at 47: women
Figure 12.4 Association between mean childhood IQ and parenthood at 47: men
In contrast, more intelligent men are no more likely to remain childless for life than less intelligent men. The mean childhood IQ of men who have remained childless is 102.2 while the mean childhood IQ of men who have become parents is 103.0. The difference is not statistically significant. Men who remained childless and men who have become parents have essentially the same mean childhood IQ. This is once again contrary to the prediction of the Intelligence Paradox.
Why Women, Not Men?
It is not clear to me why more intelligent men, who wanted fewer children than less intelligent men at the start of their reproductive careers, do not actually have fewer children. This is in sharp contrast to more intelligent women who wanted fewer children and in fact do have fewer children than less intelligent women. The data, however, allow me to rule out some possible explanations.
Some might suspect that more intelligent women have fewer children than less intelligent women, because more intelligent women are more likely to pursue higher education or more demanding careers, and, in doing so, must sacrifice and forgo motherhood. In this view, women often have to make a difficult decision between family and careers, as the latter often demands pursuit of graduate education and investment in careers during the prime reproductive years for women. As a result, women cannot often pursue both family and careers and must choose between them. In contrast, men do not have to make such a decision. They can still pursue higher education or demanding careers, and at the same time manage to have family and children.
However, the data analysis shows that this is decidedly not the reason we observe the sex difference in the effect of childhood general intelligence on lifetime fertility among the NCDS respondents. It is not why more intelligent women have fewer children while more intelligent men don’t. How do we know?
We know this because neither education nor income has any effect on the number of children women have. If the alternative explanation above is correct, then more educated women and women with greater earnings (which usually accompany demanding careers) should have fewer children than women with less education and earnings. But this is not the case. Only childhood intelligence, not educational achievement or earnings, decreases the number of children women have. Contrary to popular belief, more educated women and women with more demanding careers do not have fewer children and are not more likely to remain childless.
Another possibility is that women find intelligent men more attractive as mates. The evolutionary psychologist Geoffrey F. Miller has consistently argued that women preferentially select men with higher levels of intelligence to mate with.2 Given that, as I discuss in Chapter 7, mating among mammalian species is largely a female choice, women's preference for intelligent men as mates can potentially explain why more intelligent men may end up with just as many children as less intelligent men, despite their desire to have fewer children and to remain childless.
There appears to be some evidence for this suggestion. Net of the same control variables as above, more intelligent men are significantly more likely to have ever been married and to be currently married at age 47 than less intelligent men. One standard deviation (15 IQ points) increase in childhood IQ increases men's odds of having ever been married by 23% and the odds of being currently married by 27%. In contrast, more intelligent women are no more likely to have ever been married, although they are more likely to be currently married. It is important to point out, however, that whether they have ever been married and whether they are currently married are always controlled in all the multiple regression analyses summarized above. So more intelligent men's greater tendency to have ever been married and to be currently married is not the only reason for the absence of the effect of intelligence on men's fertility and parenthood.
At any rate, the divergent effect of childhood intelligence on completed fertility for men and women, where more intelligent women have fewer children than less intelligent women but more intelligent men do not have fewer children than less intelligent men, means, among other things, that modern British people are not very endogamous on intelligence. More intelligent men do not appear to marry more intelligent women in the contemporary United Kingdom. If they are strongly endogamous on intelligence, then the fact that more intelligent women have fewer children will necessarily mean that more intelligent men also have fewer children. This is not the case, and the only reason is that more intelligent men are not married to more intelligent women, and vice versa.