Firstborns frequently score higher on IQ and achievement tests than their younger siblings, but a new report suggests that how siblings are raised, not their birth order, is what matters when it comes to brain power.

"This study provides evidence that the relation between birth order and IQ score is dependent on the social rank in the family not birth order as such," write investigators in Friday's edition of Science magazine.

Dr. Petter Kristensen of the National Institute of Occupational Health in Oslo, Norway, and a colleague studied the birth order, IQ, and vital status of elder siblings of more than a quarter million 18- and 19-year-old male Norwegian military draftees.

The investigators focused on teasing out the biological effects of birth order from the effects of social status within the family by comparing the IQs of young men who had experienced the early loss of an older sibling and those who had not had this experience.

"In this way," Kristensen explained, "we were able to identify men who had first rank in social terms but second or third rank in biological terms and men who had second social rank in the family but were third born."

For comparison, they included in the study men born first, second, or third who had not experienced the early loss of an older sibling.

The investigators found that children raised as the eldest showed slightly higher IQs than their younger siblings. Even if a child had lost an older sibling and was raised as the eldest, their IQ was higher by an average of 2.3 points than their younger siblings.

This strongly suggests, Kristensen said, that the association between IQ and birth order is related to the social rank rather than the actual birth rank.

Interaction with the family is "the most likely explanation," Kristensen told Reuters Health. Family interaction means that the first born has the advantage of getting all the attention of the parents until the next child comes along. Later, the parents' resources have to be shared with more children.