pnas | 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 single nucleotide polymorphism
level tend to be positively correlated (homophilic). In fact, the
increase in similarity
relative to strangers is at the level of
fourth cousins. However, certain genotypes are also negatively
correlated (heterophilic)
in friends. And the degree of correlation
in genotypes can be used to create a “friendship score” that predicts
the existence
of friendship ties in a hold-out sample. A
focused gene-set analysis indicates that some of the overall
correlation in genotypes
can be explained by specific systems; for
example, an olfactory gene set is homophilic and an immune system gene
set is heterophilic,
suggesting that these systems may play a
role in the formation or maintenance of friendship ties. Friends may be a
kind of
“functional kin.” 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.
