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Born to Be Smarter

Social birth order is associated with IQ.

I always knew my older brother was smarter than I. Now, a study confirms this by associating birth order with intelligence. But, is it being the oldest or being the firstborn that is associated with higher IQ? The investigators distinguished between biologic birth order and social birth order by examining intellectual performance in men whose older siblings had died.

The subjects were 241,310 male Norwegian conscripts born from 1967 through 1976. The intellectual test administered to the conscripts is highly correlated to IQ. The researchers examined only sibships with intelligence test results and with at least one male born first, second, or third.

Analyses were adjusted for possible confounders, e.g., parents’ education. Firstborn men had a greater mean IQ than did those born second or third (103.2 vs. 101.2 and 100.0). Mean IQ was 102.9 in second-born men with early loss of an elder sibling and 102.6 in third-born men with early loss of two siblings.

Comment: Social birth order, not biologic birth order, predicted IQ. What occurs within the family influences the development of intelligence. A remaining question is the importance of this difference in IQ. On first blush, 2 to 3 IQ points do not seem clinically significant. However, commentator Frank Sulloway notes that if a new therapy produced a 13% difference (the estimated relative risk ratio) in IQ, it would "make front-page headlines."

Sulloway argues in his book Born to Rebel that those who are not firstborn also benefit from birth order, specifically in creativity and forging new paths. So, younger sibs should not be depressed — we don’t need those few points, anyway.

— Jonathan Silver, MD

Published in Journal Watch Psychiatry July 23, 2007

Citation(s):

Kristensen P and Bjerkedal T. Explaining the relation between birth order and intelligence. Science 2007 Jun 22; 316:1717.

• Original article (Subscription may be required)
• Medline abstract (Free)

Sulloway FJ. Birth order and intelligence. Science 2007 Jun 22; 316:1711-2.

• Original article (Subscription may be required)
• Medline abstract (Free)