If you've ever watched the TV show Survivor, you've no doubt noticed how crucial alliances are and how they constantly change and shift as players jockey for position and try to keep themselves from getting voted out of the game. Part of the reason an alliance of more than two people is so hard to keep together is that there's always a hierarchy of allegiance; loyalty, support, and protection do not apply equally among all. This is usually the argument one alliance uses to lure over a player from the bottom of another, bigger alliance: Being fourth from the top in our alliance, they say, is better than being fifth from the top in theirs. It's an argument that works with varying success.
So I was interested to read about a new theory called the "Alliance Hypothesis for Human Friendship," based on a study by Peter DeScioli and Robert Kurzban, psychologists at the University of Pennsylvania. The psychologists say that how you rank your best friends is closely related to how you think these friends rank you.
As Kurzban notes:

Friendships are about alliances. We live in a world where conflict can arise and allies must be in position beforehand. This new hypothesis takes into account how we value those alliances. In a way, one of the main predictors of friendship is the value of the alliance. The value of an ally, or friend, drops with every additional alliance they must make, so the best alliance is one in which your ally ranks you above everyone else as well.

Traditionally, friends have been thought of as "exchange partners" based on the "theory of reciprocal altruism." But research has shown that we don't regularly keep track of the benefits we give and receive in our close friendships, and we help close friends even when the likelihood of them repaying us is slim. The "alliance" theory of friendship is more optimistic since “it’s not what you can do for me, it’s how much you like me," says Kurzban. "In this manner even the weakest nations, for example, or the least popular kid at the party with nary an alliance in the room is set up to be paired with someone looking for a friend.”
Assuming he's right, can we use this to our advantage? Yup, the psychologists say, by 1) ranking our friends, 2) ranking them according to our position in their rankings (preferring friends who rank us higher—our more reliable allies), and 3) hiding our friend-ranking. —Heather Wax