Intelligence and family size reconsidered

Abstract

The major purpose of this study is to examine the association between the measured intelligence and fertility of over 9,000 persons who graduated high school in Wisconsin in 1957. Various measures of association are considered, including the IQ selection differential, which provides an estimate of what the generational change in mean IQ would be if, hypothetically, each child in the birth histories had the same IQ as the mean of its parents’ IQ's. This is calculated not only for graduates but also, more realistically, for the complete cohort, including dropouts. The IQ selection differential for the complete cohort is estimated to be eight‐tenths of an IQ point decline in a generation. The contribution of females to this decline is estimated to be almost five times greater than the contribution of males. The value of eight‐tenths may be viewed as an upper bound of the generational decline in mean genotypic IQ for this cohort and its offspring. An educated guess, based partly on genetic models and findings from IQ heritabil‐ity studies as well as on the above estimate of the IQ selection differential, is that the generational change in mean genotypic IQ is about one‐third of an IQ point decline for this cohort and its offspring.