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.
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