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 that 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 can set firstborns on a trajectory for success.
University of California at Berkeley researcher Frank J. Sulloway, who wrote a commentary accompanying the study, said two to three IQ points could translate to 20 to 30 points more 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 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.
Too many variables
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 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 health disadvantages.
“In medical studies, nearly all the differences favor younger children,” he said.
Making his research possible was a requirement of the Norwegian army that all conscripts to undergo an IQ test. Kristensen looked at test results of all conscripts ages 18 and 19 between 1985 and 2004.
His analysis found that firstborns had an average IQ of 103.2. The average score in the general population is 100.
What causes disparity?
With these results in hand, Kristensen then pursued a deeper question: What was the cause of the disparity among siblings? Using the same data, he looked at second- and third-born men who became the eldest in their families due to the death in infancy of 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.
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 Institute who studies the relationship between IQ, race and income.
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 five 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.
Kristensen said it was clear the results don’t apply to all families and that the chances of the first child being smarter are not overwhelming.
