Posted by: adithoughts | March 7, 2011

Human friendship

Picking best friends

You’re my best friend

A best friend is likely somebody who ranks you above all of their other friends

It has long been thought that the human cognitive processes with regards to friendship have evolved to reap benefits in trade. One’s closest friends, therefore, should leave an individual with the greatest trade surplus. However, substantial evidence exists that friends cooperate without closely monitoring their trade balance, suggesting that profit or loss balances in interactions are not the fundamental basis for friendships.

An alternative is the alliance theory of friendship. Alliances are common between nations as was the case in both World War I and World War II, and the Cold War saw the majority of the world allied to either the US or Soviet camps. The alliance theory suggests that humans, like countries, have evolved to pick friends in anticipation of future disputes, when one needs help to fight against potential rivals. This means that the value of a friendship is no longer dependent on the size of the trade surpluses, but rather on the friend’s loyalty. Loyalty can be thought of as the strength of an individual’s relationship with a friend vis-à-vis the strength of the friend’s other relationships. In particular, you are more likely to pick somebody as your best friend if the person ranks you as their number one friend over all others.

A recent paper by Peter DeScioli from Chapman University and Robert Kurtzmann from University of Pennsylvania provides evidence in support. Individuals were asked to rank and compare properties of a person’s 10 closest friends, and participant’s perception of their own standing was the strongest predictor of their ranking of friends, suggesting loyalty to be the most important criterion.

DeScioli and Kurtzmann have now teamed up with computer scientists to analyse data from 11 million MySpace profiles and found a similar result. MySpace allows the ranking of friends and this data was collected along with other demographic data . Of the 11 million MySpace profiles, 3.5 million people had their best friends in the sample. The ranking given by the this friend was compared with other demographic and popularity variables of the friend in their predictive powers on picking the person’s best friend. This allowed the testing of the alliance theory using decisions publicly made with real life consequences.

DeScioli and his team report in the Association of Psychological Sciences that a friend’s ranking as number one was a strong predictor of whether the friend would be picked as best friend. On extending the analysis from the best friend to the top 8 friends, the predictive power of rankings increased. When compared to other demographic data such as age, sex, physical proximity, number one ranking by a friend was found to be the best predictor. Two measures of popularity, number one spots in the friends ranking and number of appearances in a Top Friends list were also examined but were found to be weaker than ranking in predicting a best friend.

Predictive rank does not equal causation, and does not rule out other underlying human cognitive processes besides forming alliances. However, it does seem like a more probable explanation for human friendships in light of the present study.



  1. If I examined most of my friends in terms of “trade surplus”, I think it would look like I had made a lot of terrible decisions in terms of who to hang out with. The loyalty aspect makes a lot more sense, since a person who trades with you, but is just as likely to screw you and take everything, is going to be pretty difficult to trust. I like to think that trust is the key to any long-term “best friend”-type relationship.

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