Fertility and Intelligence - Later Research

Later Research

Regardless of the methodology employed, later research has generally supported that of Vining.

In a 1988 study, Retherford and Sewell examined the association between the measured intelligence and fertility of over 9,000 high school graduates in Wisconsin in 1957, and confirmed the inverse relationship between IQ and fertility for both sexes, but much more so for females. If children had, on average, the same IQ as their parents, IQ would decline by .81 points per generation. Taking .71 for the additive heritability of IQ as given by Jinks & Fulker, they calculated a dysgenic decline of .57 IQ points per generation.

Another way of checking the negative relationship between IQ and fertility is to consider the relationship which educational attainment has to fertility, since education is known to be a reasonable proxy for IQ, correlating with IQ at .55; in a 1999 study examining the relationship between IQ and education in a large national sample, David Rowe and others found not only that achieved education had a high heritability (.68) and that half of the variance in education was explained by an underlying genetic component shared by IQ, education, and SES. One study investigating fertility and education carried out in 1991 found that high school dropouts in America had the most children (2.5 on average), with high school graduates having fewer children, and college graduates having the fewest children (1.56 on average).

The Bell Curve (1994) argued that the average genotypic IQ of the United States was declining due to both dysgenetic fertility and large scale immigration of groups with low average IQ.

In his book Dysgenics: Genetic Deterioration in Modern Populations (1996), Richard Lynn estimated that genotypic IQ have declined by 5 IQ points in Britain between 1890 and 1980. For the United States, genotypic IQ was estimated to have declined by 2.5 IQ points for whites and 6.2 IQ points for blacks between 1890 and 1960–1964. The intelligence researcher James Flynn confirmed Lynn's assertions in a 2009 British study.

Robert Klark Graham in 1998 argued that genocide and class warfare, in particular discussing the examples of the French Revolution and the Russian Revolution, have had a dysgenic effect through the killing of the more intelligent by the less intelligent.

In a 1999 study Lynn examined the relationship between the intelligence of adults aged 40 and above and their numbers of children and their siblings. Data were collected from the 1994 National Opinion Research Center survey among a representative sample of 2992 English-speaking individuals aged 18 years. Findings revealed that weak negative correlations of −0.05 and −0.09, respectively were found. Further analysis showed that the negative correlation was present only in females. The correlation for females between intelligence and ideal number of children was effectively zero.

In 2004 Richard Lynn and Marian Van Court attempted a straightforward replication of Vining's work. Their study returned similar results, with the genotypic decline measuring at 0.9 IQ points per generation for the total sample and 0.75 IQ points for whites only.

Meisenberg (2010) found that intelligence in the US was negatively related to the number of children, with age-controlled correlations of −.156, −.069, −.235 and −.028 for White females, White males, Black females and Black males. This effect was related mainly to the general intelligence factor and was caused in part by education and income, and to a lesser extent by the more "liberal" gender attitudes of those with higher intelligence. Without migration the average IQ of the US population will decline by about 0.8 points per generation.