By Lew Harris
Jan. 9, 2001

Since Jo-Anne Bachorowski began studying the differences in the ways that people laugh depending on the sex and familiarity of the company they are keeping, the assistant professor of psychology has been thrust into the media limelight.

The research, which she conducted with graduate student Moria Smoski and Cornell psychology professor Michael J. Owren, has been the subject of articles in the Los Angeles Times, Scientific American, Elle and Self magazines, the Canadian Discovery Channel and Discovery.com. Their work has been particularly well received in the United Kingdom, where she and Owren have appeared in a total of seven interviews on various BBC programs.

In the study, they first recorded how 120 undergraduates laughed in different kinds of social pairings while they watched humorous scenes from movies.

The researchers discovered that people produce a wide variety of laugh sounds with a remarkable range of vocal pitch. In particular, they determined that individuals vary both the number and kinds of laughs they produce depending on the sex of their social partner and whether their social partner is a friend or stranger.

"We think that laughter is one of a package of subtle yet effective tools, like physical proximity and eye gaze, that people use, albeit unconsciously, to shape the emotional and behavioral responses of others," Bachorowski said.

They found that individual women, for example, produced laughs with markedly high and variable pitch when in the company of male strangers.

Other findings of the study include:

• Men's laughter is linked to the history of their relationship with their social partner. When paired with friends of either sex, men laughed significantly more than men who were tested alone or with a male or female stranger.
• Women's laughter is linked to the sex of their social partner. Females paired with a male friend produced more laughs than females tested alone, with a female friend, or with a male stranger.
• When paired with male strangers, women's laughter tends to be higher pitched, indicative of smaller body size, possibly exploiting men's propensity to be attracted to females with juvenile features.
• People have a rich repertoire of laugh sounds, with some sounding more like bird chirps, pig snorts, frog croaks and chimpanzee pants than normal human utterances.
• Laughs can be separated into three basic categories: (1) High-pitched, song-like laughs, which fit our stereotyped notions of laughter; (2) Snort-like laughs, with sounds produced primarily through the nose; and (3) Grunt-like laughs produced through the mouth.

In a second study, which is in press in the journal Psychological Science, Bachorowski and Owren asked other listeners to rate examples of the different laugh types in terms of their friendliness, sexiness, how interested they would be in meeting the laugher, whether they thought the laugh should be included in a laugh track, and the extent to which it elicited a positive emotional response.

Regardless of the rating scheme, the researchers found that listeners were more likely to rate comparatively stereotypical, song-like laughs more positively than the other types.

"These results support the notion that one important function of laugh acoustics is to influence the emotional responses of listeners," Bachorowski said.

From an evolutionary perspective Owren and Bachorowski speculate that human laughter evolved as a way to form alliances. First came the smile, which communicated a positive disposition to other individuals. Over time, however, smiles became increasingly easy to fake, so a more complex signal was needed. That is where laughter came in. Because laughter uses more neural systems and has greater energy costs, it is more difficult to fake. So, at some point, laughter supplanted smiling as an honest signal of an interest in joining forces. Their perspective on the evolution of positive emotional expressions appears in a chapter of the book, Emotions: Current Issues and Future Development, published by Guildford Press.

This research was funded by the National Science Foundation.