August 17, 2009

Human See, Human Do?

Photo of two capuchin monkeys

Imitation is the sincerest from of flattery, the old saying goes. It may also help to promote social bonds. A new study reports that monkeys prefer humans who imitate them over those who don't. Mimicry, the researchers suggest, may be an ancient behavior that sets the stage for primates to form social groups.

People sometimes unconsciously take on the body postures, gestures and mannerisms of those they encounter. Several studies have shown that humans can be swayed by this subtle behavioral imitation. Imitated people often report feeling greater rapport and empathy with those who copy them. People also may be more likely to help those who imitate them or even leave them more generous tips, research suggests. It's been unclear if this type of bonding is unique to humans.

To investigate, an international research team studied capuchin monkeys, a highly interactive species that forms strong social groups. The team included investigators from NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) and 2 Italian research institutions. The study was described in the August 14, 2009, issue of Science.

In a series of experiments, each monkey was given a wiffle ball and then paired with 2 human researchers. One investigator mimicked the monkey's behavior by poking, mouthing or pounding the ball to match the animal's actions. The other researcher adopted a different behavior—for example, pounding the ball when the monkey poked it.

During the behavior sequence, the monkeys looked longer at the researchers who mimicked them. Afterward, the animals consistently chose to spend more time near the investigator who copied them than with the other investigator. The research team interpreted this as a sign of affiliation toward the imitator.

In another set of experiments, the researchers tested how imitation affects social interactions. The monkeys were trained to take a small trinket from an investigator's hand and then return it in exchange for a food reward—a small piece of marshmallow. At first, the monkeys showed no preference for either investigator for their trinket exchanges. But after the imitation sequence, the monkeys consistently chose to interact with the imitator, although both investigators had identical rewards.

These findings suggest that mimicking behaviors can act as a type of social glue, helping to bind individuals together. Imitation may have played an important role in the evolution of both humans and some nonhuman primates, the scientists propose, by increasing group cohesion and survival.

The study may also shed light on other aspects of human health. “Researchers have known that human beings prefer the behavior of other people who subtly imitate their gestures and other affects,” says NICHD Director Dr. Duane Alexander. “Observing how imitation promotes bonding in primates may lead to insights in disorders in which imitation and bonding is impaired, such as certain forms of autism.”

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