WASHINGTON - Firstborn sons have higher IQs than their younger brothers, and their social status within the family may explain why, researchers reported on Thursday.

A study that used military draft records for 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 1.1 points on average.

Men who had been raised as the eldest, whether they were born first, second, or third, had IQs to match their first-born peers. The same was true for those raised or born second, Petter Kristensen and colleagues at the University of Oslo report 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 mens' family birth records and found families whose first-born or first- and second-born children had died before the age of one year.

Larger share of family resource?

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.

Kristensen said the findings fit with most existing theories about why merely being older might affect someone's IQ.

Various researchers have suggested that older siblings might benefit from a larger share of family resources, the process of tutoring their younger brothers and sisters, or from expectations placed on their social rank.

"Things like intellectual resources (and) stimulation from the parents to the child seem to be very important," Kristensen said in a telephone interview.

The findings swayed even skeptics of the theory that birth order affects intelligence.

"Birth order has been studied in relation to everything you can think of," said Joe Rodgers, a professor of psychology at the University of Oklahoma who was not involved in the research.

He said he was impressed that Kristensen's team was able to document a 2.3-point difference in IQ in such a large group.

"An awful lot of parents would pay money if their kids could increase IQ by two real IQ points," Rodgers said in a telephone interview.

The IQ differences were larger in brothers born into smaller families, and to married women with higher education. But the effect seems to vanish with greater age gaps between siblings, Kristensen's team wrote in the journal Intelligence.

It is unclear what the gap means for individual families, and if it can be found outside this population of young Norwegian men.

"I don't think it would surprise anyone that life is different in Norway than it is in the United States," Rodgers said.

However, "there is no reason to suspect that this should not be valid concerning women as well as men," Kristensen said.

He said he would be interested to see how siblings compare in cultures in which extended family members live together.