First-born children scored the highest on intelligence tests in a Norwegian study, likely because of social, not biological factors, researchers concluded.

The study, published this week in the journal Science, found that men whose older sibling had died — and therefore made them the eldest — scored as well on its measure of IQ as those who were born first.

This refutes the argument that prenatal factors could be responsible, said the authors.

"If the birth order effect was gestational, second-born children who are raised as the eldest would have IQ scores equal to those of other second-born children," wrote study authors Petter Kristensen and Tor Bjerkedal.

Their study found that first-borns had IQ scores generally 2.3 points higher than their peers, who came later in their families.

Studies have long suggested a relationship between birth order and intelligence and several explanations have been proposed.

This study, Explaining the Relation Between Birth Order and Intelligence, supports the view that it's how the family treats the oldest child that affects the test scores.

The data was gathered from 250,000 male Norwegian military conscripts, aged 18 and 19, between 1985 and 2004. It drew on compulsory standardized tests taken for military admission.