New research has found that children who are adopted have slightly higher IQs than siblings who remained with their biological parents. The study, published in PNAS, was designed to tease apart genetic and environmental influences on intelligence. The results suggest that the education level of the parents who raise the child can have an impact on IQ, but there is still a strong relationship between the intelligence of the child and his or her biological parents.

“Our goal in this study was not to exclude genetic explanations,” write the authors, “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 age 18-20 during the Swedish military conscription examination, which was required by law during the years covered by the study. They identified 436 cases where one male sibling had been adopted while the other sibling had remained with his biological family. They then compared the IQ test results of the siblings, while also factoring in the education levels of both the biological and adoptive families.

The adopted siblings had, on average, an IQ score 4.4 points higher than the siblings who were reared by their biological parents. How meaningful is a difference of 4.44 IQ points? On an individual level, it might make only a small difference, but it could make a large difference on a national level to things like risk perception, accidents, and productivity. That's the conclusion of Stuart Ritchie, who researches human intelligence differences and wasn’t involved in the research. “To put it another way, I don’t think I’d want to lose four real IQ points,” he told Ars.

The boost seemed to be linked to the education levels of the adoptive parents. Most of the children were adopted into families with higher levels of educational attainment, and the more years of education the adoptive parents had, the greater the increase in IQ. At its highest, this difference resulted in the adopted children having a score 7.6 points higher than their siblings.

It worked the other way around, too. Some of the children were adopted into families with lower education, and in those cases, their IQs were lower than those of the siblings reared by their biological parents. In some cases, the difference was as large as 3.8 points.

To replicate the results using a larger sample, the researchers conducted a follow-up study using 2,341 male half-siblings (sharing one parent). Again, being adopted was linked to having a higher IQ, although with a slightly lower average difference of 3.18 points.

The results tie in well with previous research, said Ritchie. “The finding that IQ is malleable by the environment isn’t 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 that adoptive parents can have for their child’s cognitive development,” he said.

The lack of novelty isn’t a criticism, he added; the authors themselves note that their results corroborate previous findings. Earlier studies have also found an effect of adoption on IQ but used much smaller sample sizes and had fewer controls. “The strong study design gives us much better evidence than we had before,” said Ritchie.

This particular research found a smaller effect than previous studies, possibly because the environmental differences between the biological and adoptive families weren’t that large, the researchers write. Some previous studies intentionally focused on children from extremely deprived backgrounds, but in the Swedish study, both groups of parents had (on average) nine to 11 years of education. In Sweden, “extremes of poverty and wealth are relatively rare,” they explain.

The results from the study fit in with previous research that suggests a large role for genetics in determining individual differences in intelligence. “Although the separated-sibling design is especially well-suited to studying the effects of the family environment, [the results are] indicative of substantial genetic effects,” the authors emphasize. But strong genetic effects don’t mean that there’s no room whatsoever for environment—these results clearly show that parenting plays a role.

What isn’t yet known is exactly what it is about the parenting that makes a difference. Parental education levels are definitely involved, but it’s not even clear at this stage whether the adoptive parents are doing something right or whether the biological parents might actually be doing something wrong. “It remains for future studies—and, critically, future studies that are genetically informative like this one—to work out exactly what it is that parents can do to boost IQ,” Ritchie said.

PNAS, 2015. DOI: 10.1073/pnas.1417106112 (About DOIs).