Friends--they are generous and cooperative with each other in ways that appear to defy standard evolutionary expectations, frequently sacrificing for one another without concern for past behaviors or future consequences. In this fascinating multidisciplinary study, Daniel J. Hruschka synthesizes an array of cross-cultural, experimental, and ethnographic data to understand the broad meaning of friendship, how it develops, how it interfaces with kinship and romantic relationships, and how it differs from place to place. Hruschka argues that friendship is a special form of reciprocal altruism based not on tit-for-tat accounting or forward-looking rationality, but rather on mutual goodwill that is built up along the way in human relationships.
I loved this book. The cross-cultural studies alone were eye-opening and worth the read. The final chapter that explains the adaptive benefits of friendships (beyond kin selection, prisoner's dilemma strategies, etc) resonate with me. Overall, the book gave me a much deeper understanding of both why we have friends and how we form friendship (across cultures).
This is an excellent complement to other book on social evolution (e.g., Dunbar, Boehm, et al).
Despite having to read this for school, it definitely gave a lot of interesting perspectives on the process of forming and maintaining friendship, as well as what into the psychology of what constitutes a friendship. Hruschka does a good job of combining studies from a large variety of cultures and taking many different perspectives into account (even giving some credence to asexuality!) but also does a good job of acknowledging the shortcomings of the studies he uses and theories he cites.