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How do we decide whether to trust somebody?

An unusual new study of college students’ interactions with a robot has shed light on why we intuitively trust some people and distrust others. While many people assume that behaviors like avoiding eye contact and fidgeting are signals that a person is being dishonest, scientists have found that no single gesture or expression consistently predicts trustworthiness.

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But researchers from Northeastern University, the Massachusetts Institute of Technology and Cornell recently identified four distinct behaviors that, together, appear to warn our brains that a person can’t be trusted.

The findings, to be published this month in the journal Psychological Science, may help explain why we are sometimes quick to like or dislike a person we have just met. More important, the research could one day be used to develop computer programs that can rapidly assess behavior in airports or elsewhere to flag security risks.

In the first experiment, 86 undergraduates from Northeastern were given five minutes to get to know a fellow student they hadn’t met before. Half the pairs met face to face; the other half interacted online by instant message.

Then the students were asked to play a game in which all the players got four tokens and the chance to win money. A token was worth $1 if a player kept it for himself or $2 when he gave it to his partner. Players could win $4 each if both partners kept their tokens, but if they worked together and traded all four tokens, then each partner could win $8. But the biggest gain — $12 — came from cheating a partner out of his tokens and not giving any in return.

Over all, only about 1 in 5 people (22 percent) were completely trustworthy and cooperative, giving away all their tokens so that each partner could win $8. Thirteen percent were untrustworthy, keeping all or most of their tokens. The remaining 65 percent were somewhat cooperative, giving away two or three tokens but also holding one or two back for security.

Both groups demonstrated the same level of cooperation. Whether the students met face to face or online didn’t change their decisions about how many tokens to give away or keep. But students who met in person were far better at predicting the trustworthiness of the partner; that suggested they were relying on visual cues.

“Lack of face-to-face contact didn’t make people more selfish,” said the study’s lead author, David DeSteno, a professor of psychology at Northeastern. “But a person’s ability to predict what their partner was going to do was greater face to face than online. There is something the mind is picking up that gives you greater accuracy and makes you better able to identify people who are going to be trustworthy.”

To find out what cues the players were responding to, the researchers filmed the students’ five-minute conversations before the game started. They discovered that four specific gestures predicted when a person was less trustworthy: leaning away from someone; crossing arms in a blocking fashion; touching, rubbing or grasping hands together; and touching oneself on the face, abdomen or elsewhere. These cues were not predictive by themselves; they predicted untrustworthiness only in combination.

And individuals intuitively picked up on the cues. “The more you saw someone do this, the more intuition you had that they would be less trustworthy,” Dr. DeSteno said.

The researchers then conducted an experiment pairing students with a friendly-faced robot, developed by Cynthia Breazeal, who directs M.I.T.’s personal robots group.

The setup was basically the same, except the students had a 10-minute conversation with the robot before they played the game. (The extra time was needed to help the student get over the “wow” factor of talking to a robot.) A woman acted as the robot’s voice, but she was unaware of its movements, which were controlled by two other people. Sometimes the robot used only typical gestures, like moving a hand or shrugging its shoulders, but sometimes it mimicked the four cues of distrust: clasping its hands, crossing its arms, touching its face or leaning away.

Surprisingly, when students saw the robot make the hand and body gestures associated with distrust, they later made decisions in the token game that suggested they didn’t trust the robot.

In questionnaires afterward, students in both groups rated the robot equally likable. But those who had unknowingly witnessed the cues associated with distrust also rated the robot as less trustworthy, compared with students exposed to only the conversational gestures.

“It makes no sense to ascribe intentions to a robot,” said an author of the study, Robert H. Frank, an economics professor at Cornell. “But it appears we have certain postures and gestures that we interpret in certain ways. When we see them, whether it’s a robot or a human, we’re affected by it, because of the pattern it evokes in our brain responses.” Dr. Frank said the study suggested that there might have been an evolutionary benefit to cooperation — and, more important, to the ability to determine who could be trusted.

“One of the interesting big questions in evolution has always been ‘Why do people do the right thing and pass up opportunities for gain when no one is looking?’ ” he said. “But if you are known to be a trustworthy person, then you are economically valuable in many situations, and it’s also valuable to be able to identify who won’t cheat.

“Life is all about finding people you can trust in different situations.”

In the following video, watch Nexi the robot interact with one of the study participants. Then, Nexi demonstrates the four cues of distrust, followed by four samples of more neutral conversational gestures.