Hyenas make friends so that they can gain access to food. Male dolphins hook up with associates to attract the ladies. And primates groom male sidekicks to help them win arguments.
So what about human friendships - are they serendipitous or driven by hidden, even subconscious, agendas?
According to a new study, people make friends based on a perception of how they are ranked within their friends' own social circle.
People need loyal allies they can call on to help them win arguments when conflicts arise, concludes the study, "The Alliance Hypothesis for Human Friendship," published last week in the journal Public Library of Science One. If a friend already has many others "ahead" of you in the ranking, there is less likelihood he will come to your defence.
"People do have different kinds of friends - a tennis partner, a colleague at work, a childhood mate - and that makes comparisons tricky," says Peter DeScioli, the study's lead author. "But when we asked people to rank their friends from closest to least close, the key was where they believed they stood in their friend's social network."
Prof. DeScioli, an evolutionary psychologist at the University of Pennsylvania, compares this dynamic to international diplomacy, in which nations court allies, rank them in order of loyalty, and get upset when they make treaties with too many other, competing nations.
"It's the same for humans, even if it's subconscious," he explains. "People are very territorial and aware of what their friends' other friendships look like, even if strategically people try to cover this up. So friendship is a story of loyalty, but also jealousy. Think of the drama and hurt feelings around who a bride chooses to be maid of honour."
Social scientists used to believe that people choose friends based on "reciprocal altruism." In other words, to exchange favours. However, recent research overturned this theory because it found friends often extend a hand without expecting anything in return.
So Prof. DeScioli and his co-author, Robert Kurzban, tested their "alliance theory" and found that the main predictor of friendship is the value of an ally who ranks you first in importance.
The researchers conducted three different studies with about 300 different men and women, and had them rank 10 friends, from closest to least close. Each participant then ranked each friend in terms of variables such as intelligence, popularity, benefits received as a result of the friendship, secrets shared, similarities, length of friendship, how often they saw one another, and how they thought their friend would rank them.
"The individuals' own perceived rank was the strongest predictor of friendship," the researchers concluded. This remained the case when they controlled for all other variables listed above. As well, there was often a strong correlation between how people ranked a friend and how they believed the person ranked them.
Prof. DeScioli has also analyzed a feature on MySpace that allows people to rank their top friends. He looked at more than 1,000 online conversations and found that many people didn't like the feature. "But it shows that rankings are a part of friendships," he says. "You care not just how much a friend values you, but how much they value you compared to other friends."
His next study will examine the impact of gender differences on the alliance theory.
He encourages people to test the hypothesis by listing five of their closest friends, ranking them from closest to least close, and then writing down how they think the friends would rank them. "The correlation will be very strong," he predicts.