Do Humans Pick Friends Who Have Similar Genetic Makeup?

Nicholas A. Christakis and James H. Fowler, in a recent paper titled “Friendship and Natural Selection,” make an interesting hypothesis: that we select friends who have similar genetic makeup as ourselves. The dataset used was the famous Framingham Heart Study. From their abstract:

More than any other species, humans form social ties to individuals who are neither kin nor mates, and these ties tend to be with similar people. Here, we show that this similarity extends to genotypes. Across the whole genome, friends’ genotypes at the SNP level tend to be positively correlated (homophilic); however, certain genotypes are negatively correlated (heterophilic). A focused gene set analysis suggests that some of the overall correlation can be explained by specific systems; for example, an olfactory gene set is homophilic and an immune system gene set is heterophilic. Finally, homophilic genotypes exhibit significantly higher measures of positive selection, suggesting that, on average, they may yield a synergistic fitness advantage that has been helping to drive recent human evolution.

So the interesting question is why would this happen? The arXiv blog goes into possible explanations:

Perhaps the genetic links are simply a reflection of this common background. Not so, say Christakis and Fowler. The correlation they have found exists only between friends but not between strangers. If this was a reflection of their common ancestry, then the genomes of strangers should be correlated just as strongly. “Pairs of (strictly unrelated) friends generally tend to be more genetically homophilic than pairs of strangers from the same population,” they point out.

There are certainly other processes that could lead to friends having similar genomes. One idea that dates back some 30 years is that a person’s genes causes them to seek out circumstances that are compatible with their phenotype. If that’s the case, then people with similar genes should end up in similar environments.

Personally, I don’t buy this:

There may be another mechanism at work. One idea is that humans can somehow identify people with similar genetic make up, perhaps with some kind of pheromone detector. Indeed, Christakis and Fowler say that some of the genes they found in common are related to olfaction, a discovery they describe as “intriguing and supportive”.

While interesting, I’m not entirely convinced of the overall findings and would be curious to see this study expand. What do you think?

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