Why Mona Lisa smiles

Women, as a rule, smile more than men, but the difference between the sexes disappears depending on the circumstances. For women, smiling is the default option. For men, the default is not smiling.

“If you don’t know what to do -- and you’re a female -- you smile because you know you’re not making a mistake. If you’re a man, you don’t smile,” says Marianne LaFrance, a psychology professor at Yale University.

In the largest analysis of smile studies ever done, LaFrance and her colleagues evaluated research involving nearly 110,000 people, finding many variations in smiling behavior.

For example, they found differences when people thought they were being observed and when they thought they were alone. When observed, women smiled more than men. When not being scrutinized, there was little difference between the sexes.

“That suggests that people behave according to what they believe is appropriate,” says LaFrance, the lead author of the study. According to some researchers, smiling is less a sign of underlying emotion than a social display meant for others.

Put a woman in front of a mirror or a shop window, though, and even alone, she’s more likely to smile. “We’re practicing what we’re supposed to be doing most of the time. We just want to make sure,” says LaFrance.

Differences in smiling all but disappear, reports LaFrance, when men and women were in the same occupation or social role. “In care-taking roles -- as a therapist or nurse, for instance -- differences were nonexistent.”

If you want to bet on who smiles most, put your money on the white, teenage girl who is with someone she doesn’t know -- she’s no doubt in default mode. Smiling differences between the sexes become larger during the teen years.

“There’s a good reason for that,” says New York psychologist Doe Lang, author of “The Secrets of Charisma” and president of Charismedia Services, a communications training firm. “Boys are trying to differentiate themselves from their mothers, and smiling is being too much like a female.”

If a boy thinks it’s a weakness to show emotion, he doesn’t show pleasure either, she says.

Such differences tend to diminish as we age.

“The comprehensiveness of this analysis is outstanding,” says Dacher Keltner, a psychology professor at UC Berkeley and director of the Berkeley Center for the Development of Peace and Well-Being. “There are studies of people smiling in social contexts, by themselves, in different cultures and at different ages.”

This study was published in the March issue of Psychological Bulletin.