Two strangers sit across from each other in a windowless room at UC Santa Barbara. They have thirty minutes to talk. Afterward, each rates the other on a battery of traits and answers a simple yes-or-no question: would you like to be friends with this person outside the lab? If both say yes, they get each other’s contact information. If either says no, they never speak again.
This is, on its face, a study about modern social behavior. Researchers measure grip strength. They ask about goals. They photograph faces and show them to strangers online. But the theory driving the design reaches back much further than any lab protocol, into a period when the wrong choice of ally could mean the difference between eating and not eating.
Adar Eisenbruch, Rachel Grillot, and James Roney, publishing in Evolution and Human Behavior,1 set out to test something that sounds almost too simple to need testing: are we choosing friends for what they can do for us right now, or for what they would have been worth to us thirty thousand years ago?
