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 2 to 3 points higher than that of younger siblings. (The average IQ is 100.)
Though that may not sound like a lot, experts said even a few IQ points could make a big difference over the course of a lifetime -- and set firstborns on a trajectory for success.
UC Berkeley researcher Frank J. Sulloway, who wrote a commentary accompanying the study, said 2 to 3 IQ points could translate to 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.
The research is the latest twist in a phenomenon 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 fill American colleges in disproportionate numbers.
Theories for the so-called birth-order effect abound: genetics, family interactions or socioeconomic factors.
After years of research, there is no consensus on the effect -- or that it even exists.
Eric Turkheimer, a University of Virginia researcher, said 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 Petter Kristensen, an epidemiologist at the University of Oslo -- and a second-oldest son -- said he did not believe in the "birth-order effect" 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 health disadvantages. "In medical studies, nearly all the differences favor younger children," he said.
Making his research possible was a requirement ofthe Norwegian army that all conscripts undergo an IQ test. Kristensen looked at test results of all conscripts ages 18 to 19 between 1985 and 2004.
His analysis found that firstborns had an average IQ of 103.2, about 2 points higher than second-born males and about 3 points higher than men born third.
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 of one or two older siblings.
He found that those men had IQs close to that of firstborns, with second-born men at 102.9 and third-borns at 102.6.
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 in the family. They are handed more responsibility after younger siblings are born and live with higher expectations from their parents.
The results supported findings from an earlier study, published in February by the journal Intelligence.
That study found the largest IQ gaps occurred in families that were relatively affluent or had well-educated mothers. The researchers were uncertain why these factors played a role.
Spacing between births also was a factor, Kristensen said. Children born less than a year apart had the greatest IQ gaps. Differences in IQ scores diminished when there were more than five years between the first and second child, he said.
Understanding the determinants of IQ has important social implications, said William T. Dickens, an economist at the Brookings Institution who studies the relationships among IQ, race and income.
A margin of five IQ points represents one-third of the wage gap between white and black Americans, he said. Identifying the factors that affect IQ could lead to ways to correct social inequities, he said.
"Raising minority children by 5 IQ points would have a huge impact on their lives. There would be a notable increase in the average income and a decrease in dropping out of schools. It would mean a lot," he said.
Some researchers remain unconvinced of a birth-order effect.
Joseph Lee Rodgers, a psychology professor at the University of Oklahoma, said results of the two Norwegian studies contradicted findings of smaller but similar studies conducted in the U.S., where different cultural factors might come into play.
"The bottom line is: These studies are not ultimately convincing until we can reconcile them with other studies," he said.
Still, Rodgers said, the Norwegian studies "raise a fascinating question."
Kristensen said it was clear the results don't apply to all families and the odds of the first child being smarter are not overwhelming.
Among brothers with unequal IQ scores, there was a 56.7% probability that the oldest brother would score higher, he said.
Sulloway also cautioned against defining firstborns as winners and everyone else as losers.
The author of "Born to Rebel," a 1996 book examining birth order, he said younger siblings develop talents and abilities not demonstrated by older siblings.
Revolutionary thinkers, such as Charles Darwin and Isaac Newton, had older siblings, he said.