London : First-born children have higher Intelligent Quotients (IQ) than their younger siblings, researchers reported after conducting a study of 250,000 men in Norway.
Researchers at the University of Oslo in Norway reviewed data collected from 18 and 19-year-old men drafted into the Norway's military between 1985 and 2004. These young men took intelligence tests as part of their compulsory military service.
They then looked at the Norwegian birth registry to determine whether these men had older or younger siblings. Medical records also indicated whether their siblings had died shortly after birth or at a relatively early age, reported the online edition of the New Scientist.
The analysis, which was published Thursday in the US journal "Science", revealed that first-born men have, on average, an IQ that is about 2.3 points higher than those who are second-born. The trend continues such that second born men have higher IQs than their third-born brothers and so on.
The researchers said the difference is due to social, not biological, factors as younger siblings have higher IQs if they are raised as an eldest child following the death of an older brother or sister.
A further review of about 600 families that included at least four children showed that the eldest sibling typically has an IQ that is 2.9 points higher than the fourth-born sibling.
Researcher Frank Sulloway at the University of California in the US calculated that a 2.3 IQ score difference means that the eldest child has a 13 percent higher chance of having above-average intelligence than the second-born in their family.
Sulloway said parents might be able to boost the IQs of younger siblings by scheduling in more individual time with them.