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T he interest in the relation between birth order and intelligence dates back to Sir Francis Galton's English Men of Science (1). Galton found more firstborn sons in prominent positions than what he attributed to chance. This was the start of numerous studies; one of the most influential was a Science publication in 1973 showing a negative association between birth order and intelligence in young Dutch men (2). Since then, sociologists, psychologists, and demographers have proposed several explanatory models (3). The most influential models have emphasized explanations relating to interactions within the family and favorable conditions for intellectual stimulation for lowbirth-order children.Several researchers have claimed that the relation between birth order and intelligence is false, confounded by factors relating to family size: Families with low-intelligence children tend to be large, and the relation with birth order is an artifact when comparisons between families are made (3). This explanation would not produce birth order effects between siblings. Thus, the demonstration of small but notable birth order effects on intelligence quotient (IQ) in large studies examining relations within families (4, 5) contradicts the idea that artifact is the full explanation.A third model claims that the relation is explained by prenatal or gestational factors. One hypothesis suggests an effect of maternal antibody attack on the fetal brain: Maternal antibody levels tend to increase by higher birth orders in a suggested mechanism parallel to rhesus incompatibility and erythroblastosis (6). It has been shown that children of mothers with autoimmune disease have an increased risk of learning disabilities [for example, (7)], but there are no empirical data to support immunoreactivity in explaining the birth order effect.Some children have different social and biological ranks in the family. One example is children who grow up in families with deceased elder siblings. A social interaction effect within the family would result in higher scores for a secondborn who had lost an elder sibling than for subjects ranked second both socially and biologically. On the other hand, if the birth order effect was gestational, secondborn children who are raised as the eldest would have IQ scores equal to those of other secondborn children.We have data on birth order, vital status of elder siblings, and IQ scores among male Norwegian conscripts (8). This gave us an opportunity to test the family interaction and the gestational explanations. We anticipated that men who had a biological rank different from the social rank would score better than males of similar birth order who had not experienced the early loss of elder siblings if the social interaction hypothesis was right, whereas similar scores would support the gestational hypothesis. Because children from families with an adverse reproductive history had a less-advantageous distribution on a number of factors associated with low IQ (8), we considered it important to adjust for those f...

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