Conducted by researchers at Virginia Commonwealth University, the University of Virginia and Lund University in Sweden, the study showed that environment plays a significant role in the development of cognitive ability in early adulthood.
Researchers used the IQ to measure the cognitive ability and compared 436 Swedish male siblings aged 18 to 20 where one was raised by biological parents while the other by adoptive parents. The study found that the IQ of the adopted males was 4.4 points higher than that of their non-adopted sibling.
IQ scores were obtained from tests conducted during inductions for national service, which was mandatory in Sweden until 2010.
The adoptive parents of adults included in the study tended to be more educated and in better socio-economic circumstances than the biological parents.
But the reverse also applied. “Those placed in homes less educated than the family of origin performed worse than their non-adopted siblings,” the study says. “Offspring placed in the best educated homes had the highest scores.”
Eric Turkheimer, U.Va. Professor of psychology said that adoption in an educated household was the most permanent kind of change and had the most lasting effects.
The findings were published in Early Edition of the Proceedings of the National Academy of Sciences.