Firstborn Sons Have Higher IQs, Norway Study Finds

Firstborn sons have higher IQs than their younger brothers, and their social status within the family may explain why, researchers reported on Thursday.

A study that used military draft records for more than 240,000 Norwegian men found that firstborns had an edge of 2.3 IQ points on their next oldest brothers, who in turn beat brothers born third by 1.1 points on average.

Men who had been raised as the eldest, whether they were born first, second, or third, had IQs to match their first-born peers. The same was true for those raised or born second, Petter Kristensen and colleagues at the University of Oslo report in the journals Science and Intelligence.

“This study provides evidence that the relation between birth order and IQ score is dependent on the social rank in the family and not birth order as such,” Kristensen’s team wrote in Science.

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And some scientists argue that birth order IQ differences arise in the womb, while others point to family interactions.

To distill potential biological effects from social effects, Kristensen’s team dug up the young mens’ family birth records and found families whose first-born or first- and second-born children had died before the age of one year.

Larger Share of Family Resources?

That was when they discovered that it was not birth order so much as growing up as the eldest of the children in a family that made the difference.

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Various researchers have suggested that older siblings might benefit from a larger share of family resources, the process of tutoring their younger brothers and sisters, or from expectations placed on their social rank.

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The findings swayed even skeptics of the theory that birth order affects intelligence.

“Birth order has been studied in relation to everything you can think of,” said Joe Rodgers, a professor of psychology at the University of Oklahoma who was not involved in the research.

He said he was impressed that Kristensen’s team was able to document a 2.3-point difference in IQ in such a large group.

“An awful lot of parents would pay money if their kids could increase IQ by two real IQ points,” Rodgers said in a telephone interview.

The IQ differences were larger in brothers born into smaller families, and to married women with higher education. But the effect seems to vanish with greater age gaps between siblings, Kristensen’s team wrote in the journal Intelligence.

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[Editor’s Note: “Explaining the Relation Between Birth Order and Intelligence,” by Petter Kristensen and Tor Bjerkedal can be read on-line here. The PDF can be read or downloaded here. Supporting material can be downloaded here.]

Negative associations between birth order and intelligence level have been found in numerous studies. The explanation for this relation is not clear, and several hypotheses have been suggested. One family of hypotheses suggests that the relation is due to more-favorable family interaction and stimulation of low-birth-order children, whereas others claim that the effect is caused by prenatal gestational factors. We show that intelligence quotient (IQ) score levels among nearly 250,000 military conscripts were dependent on social rank in the family and not on birth order as such, providing support for a family interaction explanation.

1 National Institute of Occupational Health, N-0033 Oslo, Norway.

2 Section for Preventive Medicine and Epidemiology, University of Oslo, N-0318 Oslo, Norway.

3 Institute of Epidemiology, Norwegian Armed Forces Medical Services, N-0015 Oslo, Norway.

* To whom correspondence should be addressed. E-mail: petter.kristensen@stami.no