Homozygous twins are genetically identical by definition: they come from a single egg fertilized by a single sperm, and therefore have the same genome (net of rare mutations that occurred immediately after the division). They are widely used to study to what extent traits such as personality, health, appearance are influenced by nature (genes) and how much by culture (the environment in which we grow up). And recently, research of this type has given a somewhat unexpected result: intelligence, measured in terms of IQ, would in fact be much more linked to the quality of education than to innate predispositions or abilities, written in our genes.
In the past, similar studies had always obtained the opposite result: two homozygous twins raised in different families and environments would tend to have extremely similar IQs, indicating that a large part of our intelligence would be determined by genes.
In their new study published in the journal Acta Psychologicathe neuroscientist Jared Horvath, of the English Schools Foundation Center for Research in Hong Kong, and the psychologist Katie Fabricant, of the University of Wisconsin in Madison, believe they have identified an error that has distorted the results of previous research: they had not taken into account, properly, the type of schools attended by the twins.
Different twins
The two researchers then re-examined all available data from studies that had assessed the IQ of pairs of identical twins separated at birth. The 87 pairs of twins thus obtained were re-evaluated, taking into account the type of schools they had attended growing up.
And this time, the results were very different: the differences in IQ between the twins who had attended the best schools and those who went to the worst schools reached 15 points. More or less what is observed by studying people who are not related to each other: the effect of education on intelligence would therefore appear to be predominant compared to its genetic determinants.
The search continues
However, there is a big but: finding pairs of twins who have been separated and raised in different homes is not easy, and finding them entrusted to families so different as to offer them diametrically opposed educational paths is even more difficult. In total, the research identified just 10, a number too small to draw truly reliable conclusions. According to its authors, however, it is if nothing else an indication that the debate on the nature and origin of what we define as intelligence is not yet closed, which could spur the scientific community to new efforts on the subject.
“There is still a lot of work to be done to fully map environmental influences on cognitive tests,” admit the two authors. “We hope that our work can prove to be a brick in the larger wall of knowledge that is being erected.”