This study suggests that the people we choose as friends are, on average, as genetically like us as fourth cousins.
I need to think about this. A friend of mine believes in reincarnation and thinks that we tend to be reborn into a group with friends/relations/rivals from previous lives. I’m not sure that genetically-related friends reify this faith, but if you wanted to look for evidence confirming his belief, this seems like the sort of thing that points in the correct direction.
From the article:
Beyond the average similarities across the whole genome, Fowler and Christakis looked in the study at focused sets of genes. They find that friends are most similar in genes affecting the sense of smell. The opposite holds for genes controlling immunity. That is, friends are relatively more dissimilar in their genetic protection against various diseases.
The immunity finding supports what others have recently found in regards to spouses. And there is a fairly straightforward evolutionary advantage to this, Fowler and Christakis say: Having connections to people who are able to withstand different pathogens reduces interpersonal spread. But how it is that we select people for this benefit of immunity? The mechanism still remains unclear.