Contrary to popular belief, having more children born into a family does not necessarily result in lower-IQ children. In their study, the researchers looked at data from the National Longitudinal Survey of Youth (NLSY), which gave them the opportunity to look at a large random national sample of families that included children whose academic performance had been reviewed multiple times throughout their academic careers. The NLSY originated in 1972 as a household probability sample of the nation's youth ages 14-22. For 22 years the sample followed 11,406 young people at yearly intervals. Starting in 1986, the children born to the original female respondents were surveyed every other year. The family structure measures and intelligence scores of these children provide the basic data used in this study.

The relationships among family size, birth order and intelligence have been the subjects of much earlier research. However, most of that previous research has been limited by problems of evaluating within-family models using only across-family measures. For example, family size is an "across-family" measure, while birth order is a "within-family" measure. According to the authors, earlier research on the issue of a link between birth order and intelligence lead to spurious conclusions, one of them being an apparent link between both birth order and offspring intelligence and family size and offspring intelligence. These "links", according to the authors, were caused by mistaking across-family effects for within-family effects.

"There are many good reasons why parents might consider limiting their family sizes, but the belief that, for a particular set of parents in a modern country like the United States, a larger family will lead to children with lower IQs appears to be, simply, wrong. The belief that birth order acts directly to decrease the intelligence of children born later in a given family also appears to be, simply, wrong," state the authors.

If family size does not directly affect children's IQ, what does? Numerous things, the authors' analysis suggests. Parents' IQ is an important causal source of the relationship between family size and children's IQ, because low-IQ parents have been having relatively larger families in the U.S. than high-IQ parents, but family environment and genetic heritage may also play roles in both family size and children's intelligence.

Reference: "Resolving the Debate Over Birth Order, Family Size and Intelligence". Joseph Lee Rodgers, Ph.D.; H. Harrington Cleveland, Ph.D.; Edwin van den Oord, Ph.D.; and David Rowe, Ph.D., American Psychologist, Vol. 55, No. 6. June 2000.

Contact Lead Author Joseph Rodgers, Ph.D. at: (405) 325-4591.

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