Norwegian researchers now report that it isn't a matter of being born first, but growing up the senior child, that seems to result in the higher IQ scores.
Petter Kristensen and Tor Bjerkedal report their findings in Friday's issue of the journal Science.
It's a matter of what they call social rank in the family -- the highest scores were racked up by the senior boy
-- the first born or, if the firstborn had died in infancy, the next oldest.
Kristensen, of Norway's National Institute of Occupational Health, and Bjerkedal, of the Norwegian Armed Forces Medical Services, studied the IQ test results of 241,310 Norwegian men drafted into the armed forces between 1967 and 1976. All were aged 18 or 19 at the time.
The average IQ of first-born men was 103.2, they found.
Second-born men averaged 101.2, but second-born men whose older sibling died in infancy scored 102.9.
And for third-borns, the average was 100.0. But if both older siblings died young, the third-born score rose to 102.6.
The findings provide "evidence that the relation between birth order and IQ score is dependent on the social rank in the family and not birth order as such," they concluded.