New research has found that children who have been adopted have slightly higher IQs than siblings who have stayed with their birth parents. The study, published in PNAS, is designed to separate genetic and environmental influences on intelligence. The results suggest that the educational level of the parents raising the child may influence IQ, but there is still a strong relationship between the intelligence of the child and his or her birth parents.
“Our goal in this study was not to rule out genetic explanations,” the authors write, “but rather to control for them while focusing on a natural experiment involving differences in environmental experiences.”
The researchers focused on men in Sweden who were all required to take an IQ test at the age of 18-20 during the Swedish military conscription exam, which was required by law during the years covered by the study. They identified 436 cases where a male sibling had been adopted while the other sibling had stayed with his biological family. They then compared the siblings’ IQ test results, while also taking into account the education level of both the biological and adoptive families.
The adopted siblings had, on average, an IQ score 4.4 points higher than the siblings raised by their birth parents. How meaningful is a difference of 4.44 IQ points? On an individual level it may only make a small difference, but on a national level it can make a big difference to things like risk perception, accidents and productivity. That is the conclusion of Stuart Ritchie, who studies human intelligence differences and was not involved in the study. “To put it another way, I don’t think I would want to lose four real IQ points,” he told Ars.
The impulse seemed to be related to the educational level of the adoptive parents. Most children were adopted into families with a higher level of education, and the more years of education the adoptive parents had, the greater the increase in IQ. At most, this difference resulted in the adopted children scoring 7.6 points higher than their siblings.
It also worked the other way around. Some children were adopted into lower-educated families, and in those cases, their IQs were lower than those of the siblings raised by their birth parents. In some cases the difference was even 3.8 points.
To replicate the results with a larger sample, the researchers conducted a follow-up study with 2,341 male half-siblings (who share one parent). Again, being adopted was linked to a higher IQ, albeit with a slightly lower mean difference of 3.18 points.
The results are in line with previous research, says Ritchie. “The finding that IQ is malleable by environment is not new. We know from many studies that education seems to add a few IQ points, for example. But this study is an elegant demonstration of the positive effects adoptive parents can have on their child’s cognitive development,” he said.
The lack of novelty is not a criticism, he added; the authors themselves note that their results confirm previous findings. Previous studies have also found an effect of adoption on IQ, but used much smaller samples and had fewer controls. “The strong study design gives us much better evidence than before,” Ritchie said.
This particular study found a smaller effect than previous studies, possibly because the environmental differences between the biological and adoptive families were not as great, the researchers write. Some previous studies deliberately focused on children from extremely disadvantaged backgrounds, but in the Swedish study both groups of parents had (on average) received nine to eleven years of education. In Sweden, “extremes of poverty and wealth are relatively rare,” they explain.
The results of the study are in line with previous research suggesting that heredity plays a large role in determining individual differences in intelligence. “While the separated sibling design is particularly well suited to study the effects of the family environment, [the results are] indicative of substantial genetic effects,” the authors point out. But strong genetic effects do not mean that there is no room for the environment at all – these results clearly show that parenting plays a role.
What is not yet known is what exactly makes the difference in parenting. The educational level of the parents certainly plays a role, but it is not even clear at this stage whether the adoptive parents are doing something right or whether the biological parents are perhaps doing something wrong. “It remains for future studies — and, critically, future studies that are genetically informative like this one — to find out exactly what parents can do to raise IQ,” Ritchie said.
PNAS2015. DOI: 10.1073/pnas.1417106112 (About DOIs).