Laughter in our own species, of course, is triggered by a range of social stimuli and occurs under a wide umbrella of emotions, not always positive. To name just a few typical emotional contexts for laughter, it can accompany joy, affection, amusement, cheerfulness, surprise, nervousness, sadness, fear, shame, aggression, triumph, taunt and schadenfreude (pleasure in another's misfortune). But typically laughter serves as an emotionally laden social signal and occurs in the presence of others, which led psychologist Diana Szameitat and her team to explore the possible adaptive function of human laughter. Their study, published in Emotion, provides the first experimental evidence demonstrating that human beings possess an uncanny ability to detect a laugher's psychological intent by the phonetic qualities of laugh sounds alone. And sometimes, the authors point out, laughter signals some very aggressive intentions, a fact that should—from an evolutionary perspective—motivate appropriate, or biologically adaptive, behavioral responses on the part of the listener.
Now, it's difficult, if not impossible, to induce genuine, discrete emotions under controlled laboratory conditions, so for their first study Szameitat and her colleagues did the next best thing: they hired eight professional actors (three men and five women) and recorded them laughing. This isn't ideal, obviously, and the researchers acknowledge the limited applicability of using “emotional portrayals” rather than genuine emotions. But “the actors were instructed to focus exclusively on the experience of the emotional state but not at all on the outward expression of the laughter.” Here are the four basic laughing types that the actors were asked to perform, along with the sample descriptions and scenarios used to facilitate the actors' getting into character for their roles:
Once these recordings were collected, 72 English-speaking participants were invited to the laboratory, given a set of headphones and instructed to identify the emotions behind these laughter sequences. These people listened to a lot of laugh sequences—429 laugh tracks total, each representing a randomly interspersed laugh pulse ranging from three to nine seconds in length, so that there were 102 to 111 laughs per emotion. (This took them about an hour, a nightmarish thought reminding me of those 1980s television sitcoms and focusing my attention on the peculiar laugh tracks in the background.) But the findings were impressive; the participants were able to correctly classify these laugh tracks by their often subtly expressed emotions significantly above chance.
In a second study, the procedure was nearly identical, but participants were asked a different set of questions concerning the social dynamics. Specifically, for each laugh track, they were asked whether the “sender” (that is, the laugher) was in a physically excited or a calm state, whether he or she was dominant or submissive relative to the “receiver” (that is, the subject of the laugh), in a pleasant or unpleasant state, and whether he or she was being friendly or aggressive toward the receiver. For this second study, there was no “correct” or “incorrect” response, because perceiving these characteristics in the laugh tracks involved subjective attributions. Yet, as predicted, each category of laughter (joy, taunting, schadenfreude, tickling) had a unique profile on these four social dimensions. That is to say, the participants used these sounds to reliably infer specific social information regarding the unseen situation. Joy, for example, invoked judgments of low arousal, submissiveness and positive valence on both sides. Taunting laughter clearly stood out: it was very dominant and was the only sound that was perceived by the participants as having a negative valence directed at the receiver.