Study finds the eldest scores 2.3 points higher than next sibling in line
Joseph Hall
health reporter
If you've spent a lifetime losing arguments to your older brother or sister, there's likely a good reason for it.
Chances are they really are a little smarter than you.
A study of more than 240,000 Norwegian teenagers says that first-born children are, on average, 2.3 IQ points more intelligent than the next kid in line.
And study author Dr. Petter Kristensen said in an interview that each successive child is likely to be a little less smart than the one before.
The research, published today in the journal Science, drew its data from male Norwegian army conscripts aged 18 and 19. In Norway, almost all male teenagers are required to perform military service and IQ tests are mandatory. It showed the average IQ of the first-born men was 103.2 compared with 101.2 for second-borns and 100 for third-borns.
Kristensen says earlier research showed the same intelligence differences in female siblings.
The Norwegian study also showed second-born men who had an older sibling die in infancy had an average IQ of 102.9 while those that had two elder siblings die had an average IQ of 102.6, still higher than second and third-borns.
"The study provides evidence that the relation between birth order and IQ score is dependent on the social rank in the family and not birth order as such," the researchers write in Science.
The Norwegian data does not shed light on why social rank matters, although there are several theories.
First among them is that the sheer amount of attention parents can shower on first-borns is diluted as each new child arrives, says Kristensen, a professor at Norway's National Institute of Occupational Health.
"When number two (or three) comes, there will be sharing of resources and less for each."
Those parental "resources" are not likely related to the material goods or money allotted to the eldest child, but the amount of time parents can spend on such things as reading and playing.
But parental attention cannot be the sole explanation for the elder child phenomenon since the research showed only children are less intelligent on average than first-borns in larger families, he says.
"In fact ... it's quite clear that only children score lower than first children in two or three-child families," he says. "I'm not sure what the reasons for this are, but there have been (theories that) the older child gains from tutoring the younger children."
The only-child effect may be unique to Norway, where most affluent and socially stable families – those likely to raise more intelligent children – tend to have at least two or three offspring.
But even accounting for socio-economic differences, some of the IQ deficit still exists for only children, Kristensen says. Earlier research showed the same intelligence differences in female siblings, according to the Norwegian researcher.
Birth order expert Frank Sulloway, who wrote an analysis of the Norwegian work, says the 2.3-point IQ difference between first and second born children may appear modest, but could have significant consequences.
He said, for example, that such a point spread could easily make the difference between getting into a first-tier university or having to settle for second- best.
Sulloway, a visiting scholar with the Institute of Personality and Social Research at the University of California, Berkeley, believes the IQ advantage enjoyed by older siblings is rooted in their function within the family unit.
In particular, he says, their frequent role as babysitter and teacher to younger siblings builds a mindset that they are more responsible and intelligent than their brothers and sisters and translates into the elevated IQ score.
Sulloway called the Norwegian paper the "most important" done in the field in the past 70 years and estimated its sample size was at least 50 times bigger than the next largest sibling order study.
He also said it left the theory that the IQ advantage doesn't exist "dead as a doornail."
But Sulloway pointed out that the "elegantly designed" Norwegian research averages out IQ levels over an entire population and that younger siblings can and often do score higher than older ones in many families.
