Birth Order, Family Size, and Intelligence The study of Rodgers et al. (2000):Rodgers et al. (2000): Contributor© POSbase 2008 In popular culture, birth order was believed to influence intelligence. Indeed, many studies found that birth order influenced intelligence: The older the child, the more intelligent.birth order However, most of those studies had a vital flaw (see also Ernst & Angst, 1983): Data were cross-sectional. They often assessed soldiers birth order and intelligence. Ernst & Angst, 1983

Birth Order, Family Size, and Intelligence © POSbase 2008 Therefore, birth order was not analysed within-family, but between different families. This could lead to apparent birth-order effects that are not real if increasing sibship size decreases intelligence. This has to do with the fact that the youngest in a two-child family can not be a single child; the youngest of three children family can not be from a two-child family, etc. Look at the example in the next slide:

Birth Order, Family Size, and Intelligence © POSbase 2008 Birth Order: Average IQ Number of Siblings: __________________________________________________________________ __________________________________________________________________ Average IQ: __________________________________________________________________ The more siblings, the lower is the intelligence of each child However, each sibling within a family has the same intelligence.

Birth Order: Average IQ Number of Siblings: __________________________________________________________________ __________________________________________________________________ Average IQ: __________________________________________________________________ Birth Order, Family Size, and Intelligence © POSbase 2008 Although there is no difference between siblings within the family, average IQ for increasing birth order decreases because older children weigh more in calculating the average.

Birth Order, Family Size, and Intelligence © POSbase 2008 Therefore, it is necessary to have longitudinal data. That is why the authors analyzed data from a large, national longitudinal sample where they could compare intelligence of siblings within family. Indeed, the found the pattern presented in the last slides: There were significant effects of family size, but no effects of birth order.

Birth Order, Family Size, and Intelligence © POSbase 2008 The authors concluded that this means that large families do not result in less intelligent children, as some studies suggest, but less intelligent parents make larger families. If this were not the case, we would see a birth order effect because the first child some time lives in a smaller family and should be more intelligent if family size affected intelligence. However, this was not the case. Most importantly, they found a correlation between the IQ of the mothers and family size.

Birth Order, Family Size, and Intelligence © POSbase 2008 These results have to be taken with some caution: The Parental IQ – Family Size correlation is not necessarily a biological phenomenon. It could just be fashionable in certain circles to have fewer children. If fashion in those same circles prescribed more children, the effect could turn; but this is an open question.