Firstborn sons have higher IQs than their younger brothers, and their social status within the family might explain why, researchers said on Thursday.

A study that used the military draft records of more than 240,000 Norwegian men found that firstborns had an edge of 2.3 IQ points on their next oldest brothers, who in turn beat brothers born third by an average of 1.1 points.

Men who had been raised as the eldest, whether they were born first, second, or third, had IQs to match their firstborn peers.

The same was true for those raised or born second, Petter Kristensen and colleagues at the University of Oslo reported in the journals Science and Intelligence.

"This study provides 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," Kristensen's team wrote in Science.

Their studies confirmed what many scientists had suspected for more than a century - that firstborns have an edge.

But attempts to prove the effect have been disputed, in part because the circumstances of each family are different.

To compensate for this, Kristensen's team studied brothers raised in the same families.

And some scientists argue that birth order IQ differences arise in the womb, while others point to family interactions.

To distill potential biological effects from social effects, Kristensen's team dug up the young men's family birth records and found families whose firstborn or first and second-born children had died before the age of one year.

That was when they discovered that it was not birth order so much as growing up as the eldest of the children in a family that made the difference.

(China Daily June 23, 2007)