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)