Researchers have found that firstborn children are smarter than their siblings - and the reason appears to be 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 that the IQs of firstborns were two to three points higher than their younger siblings'.
While that might not sound like a lot, experts said a few IQ points can make a big difference over the course of a lifetime - and set firstborns on a trajectory for success.
University of California, Berkeley researcher Frank J. Sulloway, who wrote a commentary with the study, said 2 to 3 IQ points could mean an added 20 to 30 points on an SAT college entrance exam.
"You go to a certain school, meet a famous professor, and the next thing you know you've gone on to medical school, made a great discovery and won the Nobel Prize," said Sulloway, who writes about family dynamics and personality development.
Year after year, more Nobel Prizes go to firstborn scientists and authors. Firstborns garner more than their share of National Merit scholarships, and U.S. colleges have disproportionate numbers of firstborn kids.
Theories for the so-called birth-order effect abound: genetics, family interactions or socioeconomic factors.
But despite years of research, there is no consensus on the effect - or that it even exists.
Eric Turkheimer, a University of Virginia researcher, said that there are just too many variables that shape an individual.
"There are millions of tiny things to control for," he said. "I'm very skeptical of the possibility of getting this worked out in a systematic way."
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.
His experience as a physician taught him that firstborns have lower birth-weights and other heath disadvantages. "In medical studies, nearly all the differences favor younger children," he said.
Making his research possible was a fortunate 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 above 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 because of 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.
Denise Gellene writes for the Los Angeles Times.