American and Swedish psychiatrists have compared hundreds of adopted children with full-blood siblings reared by their biological parents and calculated how much environmental factors contribute to intelligence. John Ross, writing for The Australian, says researchers have found that each additional level of the rearing parent’s education, when rated on a five-point scale, add approximately two points to their children’s IQ.
“Despite being demonstrably related to genetic endowment, cognitive ability is environmentally malleable,” the team reported in the journal Proceedings of the National Academy of Sciences (PNAS). “A portion of the IQ of adopted siblings (can) be explained by the educational level of their adoptive parents.”
The study, one of the largest of its kind, was made up of 436 pairs of Swedish brothers. Information was taken from Sweden’s census, two population registers, a health insurance database, and a military conscription list. Since mandatory IQ tests were conducted during inductions for national service in Sweden until 2010, those results were included in the information gathering.
There have been previous studies showing that adoption increased intelligence, adding as much as 4,4 points on average, to people’s IQs.
“Adoption into improved socioeconomic circumstances is associated with a significant advantage in IQ,” the paper says.
The latest study showed that 3.4 of the IQ points added could be caused by educational differences between biological and adoptive parents. In some cases, if natural parents had not finished school and adoptive parents had post-secondary schooling, the adopted children’s IQs were as much as eight points higher.
“Those placed in homes less educated than the family of origin performed worse than their non-adopted siblings,” the paper says. “Offspring placed in the best educated homes had the highest scores.”
The team added that in Sweden, the population rarely had extremes of poverty and wealth.
The goal of the study, according to Cathleen O’Grady of Condé Nast, was not to exclude genetic explanations, but to control them and to focus on a natural experiment involving differences in environmental experiences. The subjects of the study were Swedish men, 18-20 years of age, who were required to take an IQ test as part of the Swedish military conscription examination. The participants chosen were siblings, one of whom had been adopted, and one of whom remained with his biological family. Factoring in the education levels of both the adoptive and biological families, the IQ scores of both siblings were compared.
How important is a 4.4 IQ point difference? The team says that on an individual level, not very much. But, they continue, on a national level it could influence things like risk perception, accidents, and productivity. Stuart Ritchie, who researches human intelligence differences and wasn’t involved in the research, says:
“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.”
According to Rick Nauert of PsychCentral, researchers from Virginia Commonwealth University (VCU), the University of Virginia, and Lund University, in Sweden, as in most Western countries, there are more individuals who want to adopt compared to the number of adoptive children available.
“Therefore, adoption agencies see it as their goal of selecting relatively ideal environments within which to place adoptive children.” said joint first author Kenneth S. Kendler, M.D., professor of psychiatry and human and molecular genetics, VCU School of Medicine.
That is why adoptive parents had a tendency to be better educated and had better socioeconomic conditions than biological parents. The adoptive parents, when educated, are more likely to talk with their children at the dinner table, take their children to museums, and read to their children at bedtime.