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鈥淣ature teaches beasts to know their friends,鈥� wrote Shakespeare. In humans, nature may be less than half of the story, a team led by University of Colorado Boulder researchers has found.

In the first study of its kind, the team found that genetic similarities may help to explain why human birds of a feather flock together, but the full story of why people become friends 鈥渋s contingent upon the social environment in which individuals interact with one another,鈥� the researchers write.

People are more likely to befriend genetically similar people when their environment is stratified, when disparate groups are discouraged from interacting, the study found. When environments were more egalitarian, friends were less likely to share certain genes.

Scientists debate the extent to which genetics or environmental factors -- 鈥渘ature鈥� or 鈥渘urture鈥� -- predict certain behaviors, said Jason Boardman, associate professor of sociology and faculty research associate with the Population Program in 抖阴旅行射-Boulder鈥檚 Institute of Behavioral Science. 鈥淔or all the social demographic outcomes we care about, whether it鈥檚 fertility, marriage, migration, health, it鈥檚 never nature or nurture.

鈥淚t鈥檚 always nature and nurture,鈥� he said. 鈥淎nd most of the time it has a lot more to do with nurture.鈥�

Boardman鈥檚 team included Benjamin Domingue, research associate in the Population Program at IBS; and Jason Fletcher, associate professor of health policy at the Yale School of Public Health. Their research was recently published in the Proceedings of the National Academy of Sciences.

Early last year, PNAS published a study reporting evidence that certain shared genes might determine peoples鈥� choice of friends. Time magazine dubbed this 鈥渇riends with (genetic) benefits.鈥�

Boardman is a sociologist who spent five years studying genetics at 抖阴旅行射-Boulder鈥檚 Institute for Behavioral Genetics to bring insights of the social sciences to the natural sciences. He observed: 鈥淵ou can鈥檛 understand the spread of health behaviors -- why people smoke, why they drink, why they may or may not be obese -- unless you understand their genetic liability and also place them in the right social context.鈥�

The research team used data from the National Longitudinal Study of Adolescent Health. Boardman鈥檚 team focused on 1,503 pairs of friends in seventh through 12th grade in 41 schools. As with the earlier study, Boardman鈥檚 group found that some pairs of friends shared certain genetic characteristics.

The team tested the evidence, arguing that if genes were the driving friendship factor, genetically based friendship should emerge most often and easily in schools with the least amount of social friction. 鈥淏ut we found the exact opposite,鈥� he said.

In the most socially equal environments, genetic homophily (or love of the same) was 鈥減retty weak,鈥� meaning that friends were less likely to share genetic traits. He added, 鈥淚t was in the most unequal social environments that we saw the highest level of genetic homophily.鈥�

In a socially stratified school, 鈥淪tudents from different populations within the school may be effectively 鈥榦ff limits鈥� for friendships,鈥� the team wrote.

While applauding the revolutionary advances in genetics in recent years, Boardman said 鈥渨e have to have social scientists at the table, because we鈥檙e the ones with the data, methods and theories to characterize the multidimensional and multilevel nature of the social environment.鈥�

Scientists cannot fully understand heritable changes in gene expression unless they understand 鈥渨hat kind of schools people go to, what neighborhoods they live in鈥� and other social factors, Boardman said.

鈥淭o me, to say whether genes predict friendships without understanding the context within which these friendships may or may not occur just doesn鈥檛 tell the whole story.鈥�

The team鈥檚 research was funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the Office of Behavioral and Social Science Research at the National Institutes of Health. Fletcher is also supported by the Robert Wood Johnson Foundation Health and Society Scholars Program.

Boardman and Fletcher host an annual conference called Integrating Genetics and the Social Sciences.

Contact:
Jason Boardman
boardman@colorado.edu
Clint Talbott, 303-492-6111