From New Scientist: Firstborn children are the cleverest.

Firstborn children score significantly higher in IQ tests than their younger siblings, according to a large study of 250,000 military draftees in Norway.

The researchers say the difference is due to social, not biological, factors, as younger siblings have higher IQs if they are raised as an eldest child following the death of an older brother or sister. The findings could suggest better ways of parenting the youngest children in a family.

Petter Kristensen, at the University of Oslo in Norway, and colleagues reviewed data collected from 18- and 19-year-old men drafted into the country’s military between 1985 and 2004. These young men took intelligence tests as part of their compulsory military service.

Researchers then looked at the Norwegian birth registry to determine whether these men had older or younger siblings. Medical records also indicated whether their siblings had died shortly after birth, or at a relatively early age. [continue]

Thanks to Sarah for telling me about this article.