The First Born Child Is the Most Intelligent in the House

Who is older, Dexter or Deedee? Apparently, the answer to this question is Dexter, as revealed by a new research made on 250,000 military draftees in Norway. Firstborn children have appeared to score significantly higher in IQ tests than their younger siblings, something linked by researchers to social, not biological, factors, as younger brothers displayed higher IQs when raised as an eldest child following the death of an older brother or sister.

The team led by Petter Kristensen, at the University of Oslo, analyzed data of intelligence tests gathered from 18- and 19-year-old men drafted into the country's military between 1985 and 2004. The scientists investigated at the Norwegian birth registry if these men had older or younger siblings. They also considered if their siblings had died shortly after birth, or at a relatively early age.

The data analysis showed that firstborn men present, on average, an IQ higher on average with 2.3 points than those who are second-born. The same tendency followed for the third born brother compared to the second, and so on.

"In the US, SAT exams in reading and mathematics have a combined total of 1600 points, and on these two sections a difference in IQ of 2.3 could mean a 30 point difference in scores. This can mean the difference between getting into a premier university or second-tier university in the US," said Frank Sulloway, at the University of California, Berkeley, US.

A subsequent review of about 600 families with at least four children revealed that the eldest sibling typically presented an IQ with 2.9 points higher than the fourth-born sibling. "A 2.3 IQ score difference means that the eldest child has a 13% higher chance of having above-average intelligence than the second-born in their family.", said Sulloway.

The social birth order, not actual birth order, induced this effect: those whose older sibling died at a young age presented on average a 2.3-point higher IQ score than other second-born individuals. "Older siblings might have a higher IQ because they act as surrogate parents from time to time, tutoring their younger siblings in certain situations.", suggested Sulloway.

Other hypotheses are that parents invest more time and resources in their firstborn child, exposing him more to their sophisticated adult vocabulary. "Parents might be able to boost the IQs of younger siblings by scheduling in more individual time with them," said Sulloway.

Still, Kristensen signals that older siblings are much more at risk of having a low birth-weight and die before reaching adulthood.