What is friendship? I have always considered friendship to be a long-term and relatively low-stakes cooperative relationship. The traits that allow people to form friendships can be understood through several theories developed to understand the biological evolution of other cooperative traits; these theories include social evolution theory, game theory, and biological market theory. However many models of cooperation are actually quite simple and were developed for forms of cooperation that are dramatic (like eusociality) rather than subtle (like friendship). In fact, I think we underestimate how complex human friendships really are, because the cooperation and conflict within them is far more subtle compared to many other kinds of social relationships, like two rivals, parent-offspring bonds, or monogamous pair bonds.
A recent model by Leimar and Bshary, entitled “Social bond dynamics and the evolution of helping“, and published in the Proceedings of the National Academy of Sciences (PNAS), attempts to capture the complexity of cooperation in friendship-like relationships using vampire bats as an example. I wrote a commentary on the article, entitled, “Modeling the evolution and formation of animal friendship” and also published in PNAS. You can read it here or use this link if you lack access to the journal.
Carter Lab 