THURSDAY, June 21 (HealthDay News) -- First-born children possess IQs that are 2.3 points higher, on average, than their younger siblings, a new study contends.
This finding held true even when first-born children didn't survive and a younger child was reared as the eldest, scuttling the idea that genetics determines the difference in IQ among siblings, according to the Norwegian researchers who authored the report, published in the June 22 issue of the journal Science.
"This study really puts to an end a debate that's been going on for more than 70 years," said Frank J. Sulloway, a visiting scholar at the University of California, Berkeley, Institute of Personality and Social Research, and the author of an accompanying commentary in the journal. "The theory of biological differences is pretty much dead as a doornail."
While a 2.3 IQ point difference doesn't seem large, it translates into about a 30 percent increased chance of a child getting into an Ivy League university, Sulloway said.
However, study lead researcher Dr. Petter Kristensen, of the National Institute of Occupational Health in Oslo, noted that the difference in IQ doesn't predict what will happen to children in the future.
"This IQ difference has impact on educational potential on the population, giving first-born an advantage over later-born," he said. "However, on the individual level, this effect is so small that it gives little predictive power."
In the study, Kristensen and co-researcher Tor Bjerkedal, of the Institute of Epidemiology, Norwegian Armed Forces Medical Services, collected data on more than 241,000 male Norwegian draftees, 18 and 19 years old. The data included birth order, vital status of elder siblings, and IQ scores.
Kristensen and Bjerkedal found that the association between IQ and birth order wasn't genetic but rather related to the social birth order of the children.
"We provide evidence that suggests quite strongly that this effect is not an artifact, and that it is not linked to gestational factors -- adverse pregnancy effects," Kristensen said.
"Indirectly, it supports the theory that social support and attention within the family explain the difference. First children will not have to share this attention at first. The more children, the less attention will be provided to each child if parental resources are limited," he added.
Sulloway noted that there are several theories that might explain the difference in IQ between first-born and younger siblings. Among these is one that says that more money is spent on the oldest child, and, as family size increases, less money is available for other children, leaving them with less opportunity. "But this doesn't intuitively strike me as the explanation," he said.
Another theory holds that the first-born child gets more of the parents' attention, but Sulloway also discounts this theory.
Still another explanation is that older children teach younger children, and the act of teaching raises the IQ. "The problem with this theory is that teaching has to raise the IQ of the first-born more than it does the IQ of younger siblings, in order to produce a birth order difference," he said.
A theory that Sulloway likes is called "niche partitioning." This theory suggests that once a role in the family is filled, others have to find roles that help them compete for attention in the family.