Study: Genes Influence How We Choose Friends

Almost everybody knows by now that genes play an important role in everything from how our bodies react to nutrients to our predisposition to some cancers to whether or not we get depressed. A forthcoming study in the Archives of Psychiatry says that we can add how we choose our friends to the growing list of traits strongly influenced by genetic factors. A team of Virginia Commonwealth University researchers found that genes, alongside environment, strongly influence who we choose as friends. The researchers studied the peer groups of approximately 1,800 male twins, having each subject describe the level of social deviance among their friends, such as how many of their friends got drunk, used or sold drugs, or damaged property. The research showed that an individual's selection of friends--whether they chose to socialize with fewer or more socially deviant peers--was shaped by genetic factors. Not only did the researchers find that selection of friends has a genetic basis, but also that the influence of genes actually increased over time, as individuals gain autonomy in selecting their peers and building their social world. NEWSWEEK's Sarah Kliff spoke with lead author Kenneth Kendler, a professor of psychiatry in VCU's School of Medicine, about the role that genes play in determining our environments and our friendships. Excerpts:

NEWSWEEK: How do genes play into behavioral decisions, such as selecting our friends and creating our social world?
Kenneth Kendler:
When you're considering behavior and genetics, you can think of the brain as a transducing device, where the genes help structure the brain. The brain then influences our behavior, although environment plays a big role too. Our behavior then dramatically influences our physical and especially our social environment. We've seen that a variety of what used to be considered solely environmental risk factors, things like difficulties with marriages, rates of accidents, levels of social support, are partially influenced by your genotype. Take depression, for example. Part of the way that genes might influence your risk for depression is by influencing your temperament, by making you have somewhat more conflicted relationships and by increasing marital conflict, which then feeds back on you to increase your risk of depression.

One of the most interesting findings of your study is not just that there are genetic influences on peer selection, but that those influences become stronger over time. Why do genes play an increasingly important role as we get older?
To a large extent, as you move from childhood into adolescence into adulthood, much of what is driving you in the creation of that social world is your genes. Your family environment is pretty important when you're a kid, but becomes less important over time. When you're 8 years old, your friends are your neighbors, friends of your family, and you're pretty much going to spend time with your school group. When you move forward in time, what happens developmentally is that you have more and more capacity to shape your own world. You start making wider friendships, get a bicycle, then an automobile, then you leave home at the age of 18 or 19. You make important decisions about whether you're going to go to college or join the army. Your individual specific environment is pretty important once you leave home. More and more, your own social environment becomes your responsibility that you've created and not that of the constraint of your family when you're a prepubescent child.

As we gain autonomy over our social environments, how do our social groups change? And how do genes become more influential?
Imagine a person who is shy, likes to follow authority, very much enjoys doing socially acceptable tasks. He or she might have grown up in a family not so much like that, but by the time she's 16, she's finding a local church group of people like herself; at the age of 18 she decides to go to a Christian university. Her temperament allows her to shape more and more of her social world. By the time she's 25 she's made her own life that way. The opposite story is that you can have a hell-on-wheels 9-year-old, who thinks that nothing is more fun then getting into fights. At 10 years old, you find him by a heavily trafficked road throwing rocks at cars because he thinks that's fun. That's the kid who is going to get in trouble. He makes his own life too, but by the time he's 25 he's going to be able to shape himself much more. It's not that the temperament or genetic dispositions are not there early on. But it's the capacity you have increasing with time to be able to shape your own life. You really do get to make your own life as you grow up. That's what being an adult is in some ways.

Your study looked at the peer groups of male twins between the ages of 8 and 25 years old. What makes this an interesting stage of development to research?
Let's say you had a 14- or 15-year-old boy, and want to find out what his chances were of using substances, of committing antisocial behaviors. Review papers have always shown that one of the strongest predictors is to ask him to appraise his close friends and whether they participate in deviant activities, like using drugs, doing criminal or antisocial behaviors. For anyone who wants to understand the development of antisocial and other externalizing behaviors, this is going to be a key part of the story.

Do the results say anything about peer selection among females?
We studied males initially because male peer groups are typically more deviant. It's a little bit more informative. It would be interesting to expand it to females later. My guess is that the same processes are there. On average the peer group deviance is quite a bit lower for girls, although they do get nasty in some different ways.

What clinical applications does this research have?
There tends to be an overemphasis on assuming that [regarding personal relationships] we are sort of helpless victims of our environment. This study shows quite clearly that human development is not like that. Our environment can victimize us, but we are also very active in creating our environment, for good or ill. That just is a brute fact about human development. You can't ignore that fact and assume or make interventions on the basis of assuming that everything flows from environment to human, because it just isn't so.

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