Cooperative strangers turn a mutual profit

In the play “A Streetcar Named Desire,” Blanche Dubois declares that she has always depended on the kindness of strangers. Fat lot of good that did her.

Capuchins don’t monkey around when it comes to sharing with helpers. Living Links Center

Yet those who hope to avoid Ms. Dubois’ tragic mistakes need not wallow in a pool of distrust and selfishness. Successful social animals, from capuchin monkeys to college students, often heed the injunction that “one good turn deserves another,” even with strangers, according to two new studies.

Pairs of capuchins cooperate to obtain food even if only one of them has direct access to it, say Frans B.M. de Waal and Michelle L. Berger, both of Emory University in Atlanta. The monkey that captures the meal shares with the helper as a kind of payment for services rendered, de Waal and Berger propose.

People take beneficial exchanges even further, contend Kevin A. McCabe and Vernon L. Smith, both economists at the University of Arizona in Tucson. Even college students well versed in economic theories that treat only selfish strategies as rational often cooperate with anonymous partners to win money in laboratory games, McCabe and Smith say.

Players assigned to lead off in these games appear to try building a good reputation by giving cooperation a go, thus encouraging an exchange of favors over more selfish ploys, the researchers theorize.

The survival value of cooperative behavior in capuchins exposes the deep evolutionary roots of this essential element of human society, de Waal says. “[Capuchins’] tendency to share after successful joint efforts to obtain food may be the result of psychological mechanisms as complex as displaying gratitude for receiving help or as simple as feeling positively toward a cooperative partner.”

De Waal and Berger studied seven same-sex pairs of capuchins. Members of a pair were unfamiliar to each other. A pair entered a chamber, one animal on each side of a mesh screen. An experimenter placed two transparent food bowls, one containing apple slices, on a single weighted tray in front of the monkeys. Each monkey could reach a pull bar connected to the tray.

When the strength of both animals was required to pull the tray within reach, the monkey that got the food regularly broke up some into bits and let the partner grab it through openings in the screen. If each monkey pulled in a bowl of fruit pieces, or if only one had a bar to draw in the food-bearing tray, sharing rarely occurred, the scientists report in the April 6 Nature.

For college students, McCabe and Smith use a more complicated test procedure in which the volunteers were hidden from their partners. In their “trust game,” player 1 can opt to split $20 evenly with a partner and quickly end the game or to let player 2 choose between either a mutually beneficial or a selfish outcome. If given the chance, player 2 can cooperate (yielding $15 for player 1 and a $25 personal gain) or claim all $40, leaving player 1 penniless.

If self-interest rules, as in much economic theory (SN: 3/28/98, p. 205), then player 1 should consistently settle for half of $20 rather than trust player 2 to divvy up a larger amount.

However, among 24 undergraduate pairs playing this game a single time, player 1 deferred to player 2 in half the tests. Player 2 cooperated in 9 of those 12 pairs, McCabe and Smith report in the March 28 Proceedings of the National Academy of Sciences.

Among 28 pairs of economics graduate students, mostly male, player 1 deferred to player 2 in 21 pairs and got a cooperative response on 16 occasions. Cooperation fell when partners switched roles for a second round, but it still occurred most of the time, McCabe and Smith say.

Mutual deals that include expectations of return favors in the future constantly occur in stable societies, they assert. “This is not a sentimental proclivity but a characteristic of realistic assumptions that players make about each other based on long-term self-interest,” the researchers conclude.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.