Genes might influence the choice of friends people make. Recent research shows that people who are closely connected in social networks share certain genetic markers while carriers of other genes will tend to stay apart more often.
Psychologists who study interpersonal attraction have long known that similarity (both perceived and actual) is one of the strongest predictors of liking somebody. This applies not only to socially acquired characteristics such as beliefs, attitudes, class, interests and lifestyle, but also to friendship style, personality, temperament and physical attractiveness. As personality and temperament have a strong hereditary component, the notion that genetic factors might influence the workings of interpersonal attraction is nothing out of the ordinary. Around half of the variability in how and with whom people interact in social networks can be attributed to hereditary factors.
The study by American geneticist James Fowler has, however, attempted to show a direct, gene-to-gene link that corresponds to how people choose and maintain friends.
Subjects were drawn from two longitudinal studies that record data about genetics as well as about social connections (National Longitudinal Study of Adolescent Health and the Framingham Heart Study).
Several individual genes are known to influence human behavior. Most of them are connected with the neurotransmitters influencing mood, motivation, pleasure seeking and attention. Fowler and his team tested for six of such genes, and found that two of them were implicated in how friendship clusters are formed.
DRD2 gene is associated with dopamine receptors in the brain (some variants of that gene have been connected to alcoholism but this is likely to be a general association with addictive and pleasure seeking behaviors rather than an alcohol-specific one). Individuals whose DRD2 gene versions are similar tend to be part of the same friendship groups.
The other gene that shows an association with social networks is the CYP2A6 gene. People with particular variants of the CYP2A6 have a tendency to make social links with people whose CYP2A6 gene is of a different variant. The specific behavioral effects of this gene are unclear, though some studies associate it with openness to new ideas.
Altogether, the results of Fowler’s research suggest that human social networks might be, at least to some extent, influenced by the specific genetic characteristics of the individuals involved. Most genetic corrections in human populations have been attributed to kinship networks. However, people are just about the only species that form long-term non-reproductive associations with their friends. Fowler’s study is the first one that shows how genotypes of friends correlate outside a simple genetic similarity caused by kinship connections and population stratification.
Of the two associations discovered by Fowler’s study, the second one (indicating heterophily, or negative correlation between friendship and CYP2A6 variants) is more interesting. The positive correlations could be explained in a variety of ways that don’t include active choice, but the negative correlation strongly suggests an element of an active choice and thus the conclusion that genetic correlations in human friendship networks are not entirely due to the fact that people with similar genotypes (for example those interested in gambling or physical exercise) frequent similar locations.
Genetic homophily and heterophily in friendship formation might provide useful pointers towards understanding how human genotype can affect environmental exposures and, by implication, how environment (and particularly the social environment) influences an individual’s behavior.
Evolutionarily, these findings indicate that humans might be, to at least some extent, metagenomic in relation to other humans, and when analyzing the hereditary (and specifically genetic) influences on behavior, the friend’s as well as the individual’s genes should be taken into account.
Apart from the scientific importance, is there a practical significance to those findings? Not a huge one for daily interactions with friends, although the next time you feel inexplicably attracted to or put off by somebody, the explanation might lie in the genome.
Sources and further reading:
Smith, E. and Mackie, D. (2000). Social psychology p.426. Psychology Press. Philadelphia.
Wikipedia article on interpersonal attraction which covers the existing research well and is copiously referenced:
http://en.wikipedia.org/wiki/Interpersonal_attraction#Causes
The original article: Fowler, Settle, and Christakis. (2011). Correlated genotypes in friendship networks. PNAS 2011 : 1011687108v1-201011687. Retrieved on 24 January 2011 from http://www.pnas.org/content/early/2011/01/07/1011687108.full.pdf
Fowler, Christakis (2009). Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives. Little Brown.
Stephanie Pappas’ recent article has another account of the findings: Study: Genes Influence Who Your Friends Are. Retrieved on 24 January 2011 from http://www.livescience.com/health/genes-influence-friendships-110117.html
