Eighty-four participants’ faces were filmed while their sides were tickled from behind by a researcher. They were also filmed reacting to jokes by stand-up comedians, and while they placed their hand in icy water. Afterwards they answered questions about how they had felt in the different conditions.
Christine Harris and Nancy Alvarado found that when tickled, people showed some authentic ‘Duchenne’ smiles, in which the skin creases around the eyes, thus suggesting they really were enjoying being tickled. But this smiling wasn’t
"Ticklish smiling need have no closer a connection to mirth and merriment than crying when cutting onions has to sorrow and sadness..."correlated with self-reported enjoyment – people who said they’d enjoyed the tickling didn’t show any more of these genuine smiles than people who said they didn’t enjoy it, and vice versa. Moreover, while they were tickled, participants also exhibited facial expressions associated with pain, including wrinkling their nose and raising their upper lip. Also, in the tickling condition far more than in the comedy-clip condition, participants showed a mixture of pain-associated facial movements and smiling – what the researchers dubbed a ‘masking smile’ intended to conceal negative emotion.
“The dissociation between smiling and self-reported pleasure during tickle provides some support for the hypothesis that, in tickle, Duchenne smiles can arise as automatic responses to a physical stimulus that need not be mediated by positive affect [emotion/feeling]”, the researchers said.
This suggests that “…ticklish smiling need have no closer a connection to mirth and merriment than crying when cutting onions has to sorrow and sadness”, they concluded.
The authors also discussed possible evolutionary explanations for why we laugh and smile when we’re tickled, including that it might promote play in young primates: “…tickle may elicit discomfort in the one being tickled in order to motivate the developing primate to avoid the tickling and may, at the same time, elicit smiling to encourage the tickler to continue (thereby promoting rough-and-tumble play)”.
Harris, C.R. & Alvarado, N. (2005). Facial expressions, smile types, and self-report during humour, tickle, and pain. Cognition and Emotion, 19, 655-669.