Explaining the Relation Between Birth Order and Intelligence
22 Jun, 2007 05:04 pm
The purposes of our study were to quantify the size of a birth order effect on IQ, if any, and to provide evidence that could support or oppose prevailing theories.
in social terms but second or third rank in biological terms and men who had second social rank in the family but were third born. Altogether, these men counted 4627. In the comparison we included 236 683 men of birth order one to three who had not experienced the early loss of an elder sibling. We examined for a social rank or biological rank effect on IQ adjusting for other factors of potential influence. We found that increasing social rank was associated with decreasing IQ whereas biological rank had no effect once social rank had been accounted for. The size of the rank effect was approximately 2 IQ units per rank unit.
The Science study is supported by another paper with the same objectives, in press in Intelligence journal where we compared brothers within families. The results of this study was more or less identical with the Science paper results. Both studies included approximately a quarter of a million young men from approx. 175 000 families who had been tested between 1985 and 2004, and were based on individual linkage in national registers (most important, the Medical Birth Registry of Norway and the National Conscripts Service registry of draft board examinations).
The size of the effect of birth order on IQ is small, but highly significant on the group level. It is, however, hard to draw consequences on the individual level from this. In our comparison
between brothers, we found that if two brothers had different scores, the probability would be 57% that the elder would have the higher score and 43% that the younger would have the higher score. This difference is not very predictive in my opinion.
As for the explanations: our research is based on register information, and it may be difficult to make inferences as to causes since have no data on details on parental personal resources and how the parents raise their children. However, we have sorted out the likelihood of some
alternative explanations, leaving interaction within the family as the most likely explanation. The alternative explanations that have been proposed are that this effect is an artefact or that the effect is biological (gestational). Both these possibilities have been weakened by
our research, in my opinion. Family interaction as the remaining explanation means that the first born has the advantage of getting all the attention of the parents until the next child comes. Later, the parents' resources have to be divided between two etc. Also the tutoring
effect that is an element in one of the family interaction theories (Zajonc's confluence model) may matter.
Petter Kristensen and Tor Bjerkedal, Explaining the Relation Between Birth Order and Intelligence, Science 22 June 2007: Vol. 316. no. 5832, p. 1717