Study finds firstborn children are smartest

Wading into an age-old debate, researchers have found that firstborn children are smarter than their siblings – and the reason is not genetics but the way their parents treat them, according to a study published today.

The study of 240,000 Norwegian men in the journal Science found the IQs of firstborns were two to three points higher than their younger siblings.

While that may not sound like much, experts said even a few IQ points can make a big difference over the course of a lifetime – and set firstborns on a trajectory for success.

The research is the latest twist in a phenomena that scientists have long noticed but have been at a loss to explain.

Year after year, more Nobel Prizes go to firstborn scientists and authors. Firstborns garner more than their share of National Merit scholarships, and American colleges are filled with disproportionate numbers of firstborn kids.

Theories for the so-called birth-order effect abound: genetics, family interactions and socioeconomic factors.

But despite years of research, there is no consensus on the effect – or that it even exists.

Lead author Dr. Petter Kristensen, an epidemiologist at the University of Oslo – and a second-oldest son – said he didn’t believe the “birth order effect” was real when he started his research, which was originally aimed at assessing the validity of IQ tests.

Making his research possible was a requirement of the Norwegian army that all conscripts undergo an IQ test. Kristensen looked at test results of all conscripts ages 18 or 19 between 1985 and 2004.

His analysis found that firstborns had an average IQ of 103.2, nearly three points higher than second-born males and four points higher than men born third. The average score in the general population is 100.

With these results in hand, Kristensen then pursued a deeper question: What was the cause of this disparity?

Using the same data, he looked at second- and third-born men who became the eldest in their families due to the death or one or two older siblings.

He found that those men who had become the eldest had IQs of 102.9, nearly identical to the IQs of firstborns.

The findings suggested that the mechanism behind the birth-order effect is not biological but related to social interactions within families.

He surmised that older children are showered with attention early in life and treated as leaders within the family. They are handed more responsibility after younger siblings are born and live with higher expectations from their parents.