A new study conducted in Norway seems to prove that the order in which children are born influences their IQ, with a slight advantage for first born ones.

The Norwegian study was based on data gathered from more than 250,000 military draftees, which by law are obliged to undergo psychological evaluation at the age of 18. The intensive data analysis concluded that firstborn men have an IQ slightly higher (about 2.3 points at IQ tests) than their younger siblings. Researchers lead by Petter Kristensen, at the University of Oslo in Norway, investigated Army records of military draftees’ results at the compulsory evaluations between 1985 to 2004.

Moreover, the pattern extends repetitively to younger siblings: second born men are “smarter” than third born men and so on.

According to Frank Sulloway, at the University of California, Berkeley, US, cited by New Scientist, a difference in IQ of 2.3 could mean a 30 point difference in scores obtained at the famous SAT exam (the SAT has a combined total of 1600 points).

A more interesting finding is related to the influence of deceased siblings: after sifting through the Norwegian birth registry to determine whether tested men had older or younger siblings who had died shortly after birth, or at a relatively early age, researchers found that men with such experience had roughly the same IQ as the first born children.

Sulloway explains that older siblings might have a higher IQ because they act as surrogate parents from time to time, tutoring their younger siblings in certain situations.

Another possible explanation is that parents have more time and resources to invest in their firstborn child. As a result of this, the eldest child usually has the most one-on-one time with its parents, and receives greater exposure to their sophisticated vocabulary.