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This caused considerable alarm, and a number of studies were undertaken both in England and America in the early decades of the 20th century. Schoolchildren's IQs were found to correlate negatively with their number of siblings, which seemed to confirm fears of dysgenic fertility, but this conclusion was questioned because there was no way to know the IQs of the childless. Later, some U.S. studies of adult IQ and number of offspring reported negative correlations, but other similar studies found no correlation. However, the samples used in all these studies were not representative of the U.S. population as a whole - they were restricted either in terms of race, birth cohort, or geographical area. So by mid-to-late 20th century, there was still no definitive answer to the question of dysgenic fertility. Then in 1984, Frank Bean and I had the good fortune to discover an excellent data set, the General Social Survey (GSS), to test the hypothesis. It included a short vocabulary test devised by Thorndike to provide a rough grading of mental ability which was ideal for our study. The GSS had interviewed a large, representative sample of the U.S. population whose reproductive years fell between 1912 and 1982, yielding data which provided the unique opportunity of an overview of the relationship between fertility and IQ for most of the 20th century. In all 15 of the 5-year cohorts, correlations between test scores and number of offspring were negative, and 12 of 15 were statistically significant (Van Court and Bean, 1985).
