Why season of birth is related to childhood intelligence

Countless studies have found that children’s intelligence appears to be related to the time of year they were born in. Some investigators have argued this is because seasonally varying environmental factors like temperature and infections can affect brain development. But now Debbie Lawlor and colleagues have analysed data from 12,150 children born in Aberdeen between 1950 and 1956, and they’ve concluded that the effect of season of birth is almost entirely explained by the age children happen to be when they start school.

Reading ability at age 9 and arithmetic ability at age 11 were both related to season of birth (children born in late Winter or Spring performed better), but this association virtually disappeared once age at starting primary school and age relative to class peers were taken into account. That is, season of birth was only related to later intelligence because it affected the age children started school, with those who started school younger or older than the average tending to score less well on later intelligence tests.

By contrast, the outside temperature when the children were conceived, during gestation, and at their birth, had no independent association with their later intelligence.

“We have found weak season of birth effects on some aspects of childhood intelligence, which appear to be explained by differences in age at school entry and/or age relative to peers”, the researchers concluded.

However, the story isn’t entirely straightforward. The researchers predicted that children who spent less time at primary school would perform less well on subsequent intelligence tests. Instead, they found the opposite pattern. “It is possible that those who had least time in primary school but most time at home were in fact given extra tuition by their parents”, they surmised.

Lawlor, D.A., Clark, H., Ronalds, G. & Leon, D.A. (2006). Season of birth and childhood intelligence: Findings from the Aberdeen Children of the 1950s cohort study. British Journal of Educational Psychology, In Press. DOI: 10.1348/000709905x49700.
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