We are all partial to family. Our calls, visits and gifts are proximately driven by family affection, and ultimately by shared genes. Conversely, business relations with genetically-unrelated business colleagues are explained by standard 'reciprocal altruism' theories first outlined by Robert Trivers.
Friends are a problem, though. Why do we invest significant personal capital in people who are not kin relations? Friends get a level of personal commitment which extends far beyond that which lubricates routine business or social transactions.
Tooby and Cosmides, pioneers in evolutionary psychology, suggested it was a form of investment. Favours in the favour bank for when you really needed a favour back. This certainly has the ring of truth, but how to select the candidates for friendship? - After all, friendship is underpinned by emotion, not (just) by rational calculation.
In a recent article*, Krebs suggests that friendship is a 'mis-firing' of kin-preference mechanisms. Animals cannot directly detect the genes of kin conspecifics, and need proxies such as appearance and geographical proximity. Many friends resemble each other and friendships bloom via close proximity. The language of friendship is the language of kin.
This suggests a number of research issues. Whether friends mostly inhabit sibling-like relationships, or whether other familial relationships are also expressed in friendship modes. And whether having siblings (and of which gender) correlates with the number and type of friends (controlling for personality variables).
A final thought. Real friendship involves significant personal investment and costs - as they say, it's only when you're down and out that you find out who your real friends are. Most people would be horrified to discover just how few friends they actually had.
* Donald Krebs, 'An Evolutionary Reconceptualisation of Kohlberg's Model of Moral Development', chapter 9 in "Evolutionary Perspectives on Human Development" 2nd Edition. Robert L. Burgess & Kevin MacDonald (Eds.). Sage Publications, 2005.