On the possibility of the reemergence of a dysgenic trend with respect to intelligence in American fertility differentials

This paper examines the relationship between IQ and fertility in a sample of men and women aged 25–34 as of the late 1970s. This sample is of unusual interest for two reasons: (1) it is a national probability sample, representative of the non-institutional civilian population of the U.S. as a whole, and (2) it is for a post-World War II cohort. Most previous studies of the IQ/fertility relationship have employed nationally unrepresentative samples of cohorts born in the pre-war period, 1910–1940. The bias, in both time and place, of the samples used in these studies has not been adequately grasped by those who cite them as evidence of a eugenic trend with respect to intelligence.The major finding of these previous studies was that the IQ/fertility relationship is slightly positive, contrary to expectations though confirmatory of the so-called Eugenic Hypothesis. My working hypothesis is that this finding is special to the cohort chosen for study, i.e., one whose child-bearing took place during a period of rising birth rates (cohorts born between 1910 and 1940 were largely responsible for the baby boom after World War II). That is, in periods of rising birth rates, persons with higher intelligence tend to have fertility equal to, if not exceeding, that of the population as a whole. In periods of falling birth rates, the opposite is the case, according to my thesis.This thesis is generally supported by the data set described above. Fertility differentials to date within the post-World War II cohort, which entered its reproductive years during a period of falling birth rates, show a negative relationship between intelligence and fertility. The relationship is less negative for white men than for white women and for white women than for black women (black men are omitted from this study due to deficiencies in the data). The stated intentions of this cohort with respect to future fertility, if realized, will moderate the degree of this relationship, particularly for whites, but not change its sign.