Is he trustworthy? Forget what the songs say about his kiss, and check out the breadth of his cheekbones.

Men with wider faces are not only perceived as untrustworthy, they may deserve the reputation, according to a new study published in the journal Psychological Science.

In a computerized game, broad-faced men were more likely to exploit others for personal monetary gain, explained lead researcher Michael Stirrat of the University of St. Andrews in Scotland. And other players expected this behavior. The study did not address female trustworthiness.

A growing body of science is showing that facial configuration — placement of the eyes, width of the cheekbones and so on — provides clues to a person's personality, including likelihood to be extroverted, conscientious and, now, trustworthy. While this analysis is not unfailing, it works slightly more often than not.

The trust game

During the study, a participant was shown an expressionless photo of a fellow player's face at the start of each game. The participant then had to decide whether to take an immediate pay-off or entrust the money to the person in the snapshot — who, in turn, could decide to either, cooperate and help both players make more money, or take the cash and run.

Participants were more likely to entrust money to people with narrower faces, a characteristic measured by dividing the face width by the distance between the upper lip and upper eyelid. Photos of all participants were analyzed when the games were completed, allowing researchers to see a correlation between slim faces and actual trustworthy behavior.

In another experiment using digitally manipulated photographs, people chose between wide and narrow versions of the same face. Out of 285 participants, 165 said the narrower faces looked more worthy of trust.

"The effect is not huge, but there is a significant bias for trusting narrow male faces," Stirrat said. "And we are biased in the right direction."

Testosterone and wider faces

While wide-faced men were more exploitive in this study's context, "it is difficult to generalize to real life," Stirrat said.

Other experiments have shown that men with wider faces tend to be more aggressive — broad-faced hockey players spend more time in the penalty box, for example — and are perhaps more dominant. They are also more likely to use aggression for altruistic means by, say, attempting to punish people who steal or break other societal rules.

Here's how the researchers figure the link between facial width and trustworthiness might work: During male adolescence, a surge in testosterone prompts bone growth not only in the spine and limbs, but also in the face. Therefore, a wide face may be a sign of an overall bigger man, one who can get away with being more aggressive and less cooperative. Slighter men might act in a more civilized, and trustworthy, way — out of necessity.

Narrowness is only one aspect of an honest face. We are also more likely to judge someone as trustworthy — rightly or wrongly — if they have faces that are attractive, babyish, similar in appearance to our own kin, or exhibit good health. It makes good evolutionary sense for people to trust their own families and those who exhibit superior genes; even if they are let down, the survival of their own genes is maximized.

In most cases, it pays to be trusting, Stirrat said, because people who feel trusted are more likely to behave in a trustworthy way.