Political aspirations also seem to guide friendships among the male Assamese macaques Macaca assamensis, which are native to Thailand. For this social primate, dominance is the main factor that allows a male monkey access to females, and thus leads to reproductive success. In one study carried out in 2010, a monkey began ranking third in the group. Despite his relatively large size and good physical condition, he wasn't very good at making friends with the other male monkeys. It wasn't long before he tumbled to sixth position in the social hierarchy and lost his reproductive advantage. By the end of the observation period, he had fallen even further to eighth.
Could reputation protection – rather than similarity as Plato or Aristotle thought, or reciprocation as evolutionary biologists have argued – best explain the friendship riddle? In an experiment conducted by psychologists Peter DeScioli and Robert Kurzban in 2009, human participants created a list of their ten closest non-family friends, and ranked them according to closeness. They were then asked to imagine that they had one hundred points to distribute among those ten friends.
When the experiment participants were told that their distributions would be public knowledge, they doled out points fairly. Each friend received, on average, ten points. However, if the participants were told that their distributions would remain confidential, their allocations were less uniform. The best friend got the most points, followed by the second best friend, then the third, and so on. As social creatures with reputations to maintain, humans are acutely aware of the way that their behaviour might be viewed by others. So people rewarded their closest friends when they could get away with it, but strived to appear fair when under public scrutiny.
DiScioli and Kurzban use political examples to explain the complex nature of friendship, rather than simpler economic or geographic factors. They point out that despite the fact that the US traded with China over three times more than with the UK in 2006, the UK is far more likely to be described as a "friend" of America. They suggest that if "friendships are like international alliances, then friendship will not be well-explained by exchanges of benefits."
Friendships might serve as a strategic mechanism for maintaining a support system in advance of potential future conflicts. "Human conflicts are usually decided," they explain, "by the number of supporters mobilized on each side (rather than strength or agility)." So perhaps friendship only seems a riddle because if we were explicit about the transactional nature of our alliances, their strength would falter. In other words, we might like to make grand claims that friendships are without agenda, but that doesn’t necessarily mean this is the case.