Sociology: You Are Who You Know

Scientific debate often focuses on whether “nature” (taken to mean genetic inheritance), or “nurture” (parenting, schooling, and other aspects of our environment) has more influence on who we become. But a study by Princeton researchers on genetics and smoking shows that the two concepts are not as distinct as we may believe. In fact, other people’s genes — their nature — form an integral part of our own nurture. 

The study shows that a teenager’s likelihood of smoking is influenced by the genes of their classmates, says Ramina Sotoudeh, a sociology Ph.D. student who led the study. Nature and nurture are “not separable,” she says. “They’re not distinguishable from one another.” 

Research has shown that genes shape how we react to our environment, and that our environments, including the culture and politics around things like mating and migration, shape how our genes are mixed in the first place. But, this study, says Sotoudeh, considers “how genes make up the environment.” 

This so-called “metagenomic” or indirect genetic effect, has long been known in animals, but the first such effect was measured in humans two years ago, when researchers in Iceland found correlations between the genes that parents didn’t pass to their children and those children’s school performance.

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Nature and nurture are “not separable,” says Ramina Sotoudeh GS

Courtesy Ramina Sotoudeh GS

By examining patterns of teen smoking, an exemplar of peer pressure, Sotoudeh and her co-authors — Princeton sociologist Dalton Conley and UNC-Chapel Hill sociologist Kathleen Mullan Harris — found that not just parents’ genes, but peers’ genes are also in play. The researchers combined data from Add Health — a study that holds genotypes and survey data about smoking, friendships, and more for some 10,000 seventh- to 12th-graders — with previously measured associations between smoking and a long list of genetic variants as measured by an international team known as the Tobacco and Genetics Consortium. They found that the more a student’s friends and classmates had “smoking-associated” genetic variants, the more likely that student was to smoke. 

If the effects were observed only within friend groups, it could have been chalked up to smokers befriending smokers, says Sotoudeh, but seeing these correlations within essentially randomized grade cohorts suggests that genes do indeed play a role. 

As for how one teen’s genes can influence another to smoke, Sotoudeh says there’s “suggestive evidence,” but no certainty, that it’s the “contagious” act of “smoking itself,” rather than some other behavioral influence. 

Next, Sotoudeh hopes to explore in depth “the complex ways these effects unfold at the individual level,” but says that finding indirect genetic effects in schools means that scientists should be looking at many ways genes contribute to our environment in other facets of life, too. “It’s part of the reality that we live in.”