THURSDAY, June 21 (HealthDay News) -- Older children have a higher intelligence quotient (IQ) score than their younger siblings not based on their actual birth order but whether they were raised as the eldest, researchers report in the June 22 issue of Science.
Petter Kristensen, M.D., Ph.D., from the National Institute of Occupational Health, and Tor Bjerkedal, M.D., Ph.D., from the Norwegian Armed Forces Medical Service, both in Oslo, Norway, examined the effect of birth order on the intellectual performance score of over 240,000 Norwegians aged 18 to 19 years.
The researchers found that the mean IQ scores of the eldest siblings were higher than those of their next youngest siblings by a mean of 2.3 points. This was related to social order, where a child is raised as the eldest if they have lost their older siblings, rather than actual birth order.
"Thanks to the new results, we no longer need to wait for truly persuasive data to justify those theories that consider birth-order differences in intellectual performance to be a within-family phenomenon," Frank J. Sulloway, Ph.D., of the University of California, Berkeley, writes in an accompanying editorial.
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