A negative relationship between intelligence and fertility in the United States has been described repeatedly, but little is known about the mechanisms that are responsible for this effect. Using data from the NLSY79, we investigate this issue separately for Blacks, non-Hispanic Whites and Hispanics. The major findings are: (1) Differential fertility would reduce the average IQ of the American population by up to 1.2 points per generation in the absence of migration and environmental changes; (2) About 0.4 points of the effect is caused by selection within racial and ethnic groups, and the rest is caused by between-group selection; (3) Differential fertility by intelligence is greatest in Hispanics and smallest in non-Hispanic Whites; (4) The fertility-reducing effect of intelligence is greater in females than males; (5) The IQ-fertility relationship is far stronger for unmarried than married people, especially females; (5) High intelligence does not reduce the desire for children; (6) High intelligence does not reduce the likelihood of marriage; (7) Education is the principal mediator of the IQ effect for married women.
[Gerhard Meisenberg, Anubhav Kaul. Effects of Sex, Race, Ethnicity and Marital Status on the Relationship between Intelligence and Fertility. Mankind Quarterly. Washington: Spring 2010. Vol. 50, Iss. 3; pg. 151, 37 pgs.]
IQ dysgenics in the United States
Richard Hoste recently wrote about an analysis of NLSY data by Gerhard Meisenberg published in the journal Intelligence showing a negative relationship between IQ and realized fertility. Here's the abstract of a related paper from Meisenberg published in Mankind Quarterly: