Statistically speaking, more educated people tend to weigh less. That correlation alone, though, doesn’t really tell you much—you could make a parlor game out of coming up with plausible explanations. Maybe the reason is that more educated people have access to healthier foods. Maybe it’s because people who are bullied about their weight are more likely to leave school. Or maybe the people who can afford college tuition and the people who can afford gym memberships are one and the same.
In 2015, a study in Nature Genetics introduced a surprising new possibility: Perhaps weight and education are so intimately connected because they share some of the same genetic roots. Using enormous collections of genetic data, the study’s authors searched for pairs of traits that were correlated with the same genes. For each pair they calculated a metric called “genetic correlation,” which quantifies just how similar the whole set of genes linked to one trait is to that linked to another trait. A smattering of trait pairs popped out as having significant genetic correlations, among them body mass index (BMI) and years of education—as well as more obvious pairs, like depression and anxiety, or type 2 diabetes and blood glucose levels. (Researchers have since tried to explain the apparent genetic link between weight and education by suggesting that people who are genetically predisposed to be better decision makers, and are presumably successful in the classroom, are more likely to adopt healthy lifestyles.)
Compared to simpler, behavioral explanations, such genetic explanations might sound far-fetched. But the data would seem to offer few other alternatives. Genes, after all, have an unquestionable primacy. If the same genes are associated with both education and BMI, it stands to reason that those traits must have intertwined biological roots.
Now, a new study in Science shows that this idea is illusory. It suggests that geneticists must also consider what comes before people’s genes: their parents. Even if two traits are statistically associated with the same genes, they may not have any true genetic overlap: That same pattern can also appear if people with those traits tend to mate with each other. (This is called “cross-trait assortative mating.”)
For example, people with many years of education, who are likely to be of a higher social class, tend to seek out partners who display markers of social standing like a low BMI, and vice versa. Their children will then have genes linked to both high education and low weight. If this happens repeatedly across a population, the two traits will appear to share some of the same genetic causes, because the traits and genes will co-occur so frequently. In reality, they will have been inherited from different sides of the family.
Genetic correlations have become a popular tool because of what they seem to suggest about the underlying biology of a pair of traits, says Richard Border, a postdoctoral scholar in neurology and computer science at the University of California, Los Angeles and the study’s lead author. But cross-trait assortative mating challenges such inferences. “It is basically a way of breaking that logic,” Border says.
Border and his colleagues are not the first to raise the possibility of spurious genetic correlations. When designing studies, geneticists can control for the effects of factors like parental traits and childhood environment by comparing people who have those things in common—that is, siblings. Earlier this year, statistical geneticist Laurence Howe and a team of researchers did just that. When Howe compared siblings with each other, he observed no genetic correlation between BMI and years of education. Somehow, it was parents, and not genes themselves, that had made weight and education seem genetically connected.