A best friendship might have little to do with common interests, longevity, or even proximity - it might just be the mutuality.
This is the conclusion Penn psychologists Peter DeScioli and Robert Kurzban reached in their recent study exploring the cognitive aspects of human friendship.
In a series of question-and-answer interviews, the two researchers asked subjects to rank their ten best friends based on a number of criteria, which ranged from the number of shared secrets to the age difference.
According to the paper's results, benefits received from the friendship and similarities were both shown to be important properties when determining their friends' ranks.
However, the most significant criterion was found to be the subjects' own perceived ranks of what they would be on their friends' lists.
The finding stayed consistent whether the subjects were drawn from Penn undergraduates, Rittenhouse Square park visitors or an online community, giving weight to DeScioli's and Kurzban's new hypothesis, the "alliance hypothesis."
As explained in the study, friendships are theorized to be forged much in the same way that countries form alliances.
Derived from ideas in game theory and international relations, the alliance hypothesis suggests a loyal partnership-like friendship rather than one solely based on the mutual exchange of goods and care.
Many best friends were found to not only forget to keep track of such exchanges but also often be willing to lend money to the other person, even when they were not guaranteed timely repayments.
A press release in Science Daily called such alliance friendships a more "optimistic" view of friendship because it lessens the importance of traded benefits - supported in older studies - and puts more emphasis on the emotional relationship.
Though the two researchers have not previously worked on this issue before, DeScioli wrote in an e-mail, future research is anticipated, beginning with a second study that will be submitted for publication soon.
"If the alliance hypothesis turns out to be correct, then I think it will help us better understand jealousy and exclusivity in friendships," DeScioli wrote.
More specifically, DeScioli added, this knowledge would help improve the development of children's social relationships by applying it to the school setting.
"Many children suffer from social exclusion at school," DeScioli wrote, "and a better understanding of friendship in terms of alliances might point to ways that jealous and exclusion can be reduced."
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