Parents' education levels may account for difference, researchers suggest
WebMD News from HealthDay
By Tara Haelle
MONDAY, March 23, 2015 (HealthDay News) -- Adopted children tend to have a slightly higher IQ than siblings who remained with their biological parents, a recent study found.
The difference between siblings -- equivalent to about four IQ points -- appears to stem from higher average educational levels in adoptive parents, according to the researchers.
"The more educated the adoptive parents are, the bigger the advantage for the child," said study co-author Eric Turkheimer, a professor of clinical psychology at the University of Virginia in Charlottesville. "Even in the presence of genetic differences among people, improving the environment helps children's cognitive ability."
However, this study was only designed to find an association between intelligence and adoption status. It couldn't prove whether adoption actually caused higher IQ scores.
The research was published online March 23 in the Proceedings of the National Academy of Sciences.
The authors analyzed two large data sets of male siblings in Sweden. All males in that country are required to take an IQ test between ages 18 to 20 as part of the country's mandatory military service.
IQ is a measure of individuals' memory and thinking ("cognitive") ability. The median IQ score is 100, which means half have a higher score and half have a lower score, Turkheimer explained.
The first set of more than 400 pairs of full male siblings revealed that those who had been adopted had an average IQ score of nearly 97. Those who were raised by their biological parents had an average IQ of 92.
After accounting for other factors, the researchers determined that being adopted accounted for an average increase of just over 4 IQ points. Four points roughly equates to moving up 10 percent in cognitive ability compared to the rest of the population, according to Turkheimer.
"An increase of 4 IQ points is certainly substantial," said Marinus van IJzendoorn, a professor at Leiden University's Centre for Child and Family Studies in the Netherlands.
In his own research, van IJzendoorn has compared the IQs of children adopted from orphanages to those not adopted from the orphanage. "We found that if adopted children come from bad orphanages, the increase in IQ might even be 15 or more IQ points compared to the peers left behind, which is the difference between marginally mentally delayed versus normal development."
source : Adopted Kids' Average IQ Higher Than Non-Adopted Siblings: Study