You're at a surprise party. For a split second, the face of the birthday boy or girl contorts with - shock? Joy? During intense emotional experiences, there's a fleeting moment when expressions of pleasure and pain are hard to distinguish.
In fact, others read intense emotion more effectively by looking at a person's body language than by watching facial expressions, a study suggests.
Most studies of facial cues rely on a set of stylised, recognisable expressions - perhaps made by actors in photographs. The actors make expressions meant to be obvious enough to translate across cultures: anger, disgust, fear, joy, sadness and surprise. But these stylised images don't necessarily reflect the expressions that people make in the real world, says Dr Hillel Aviezer, a neuropsychologist at the Hebrew University of Jerusalem and lead author of the new study, published in Science.
Moreover, when emotions get particularly extreme, people undergoing fleeting peaks of intense pain, joy, grief, or anger look surprisingly similar, Aviezer says. From the face, at least, ''when you compare extreme pain to extreme pleasure, you really can't tell them apart''.
And yet most people are rarely confused about whether someone is experiencing grief or joy.
To figure out what tips us off, Aviezer and his colleagues showed photos of professional tennis players to 45 Princeton University students, randomly divided into three groups of 15. Each tennis player had just won or lost an important match and the participants rated the players' contorted facial expressions from negative to positive on a scale from one to nine.
One group of participants looked at head-to-toe photos of the players, the second group looked at only the players' bodies and the third group looked at only their heads.
Only the final group had trouble making the correct identification, suggesting that facial expressions alone didn't tell them whether the players were joyous or in despair.
With the help of photo-editing software, the team then switched the winning players' heads with those of losers. To keep participants from noticing the trick, they shuffled the doctored photos with similar images. Participants still labelled winners or losers according to the players' posture, not their facial expressions.
In interviews conducted after the study, the researchers learnt that cues such as whether a hand was open or clenched were more important than facial cues in interpreting expressions. Yet, in a separate experiment that asked 20 participants to state whether they would use body language, facial expressions, or both to evaluate emotion, 80 per cent believed that they could judge the full body photos by facial expression alone, Aviezer says.
That result underscores our bias towards faces and how little we credit body language, he says.
To see if bodily gestures were more expressive in other contexts, the researchers performed a similar experiment with photographs of people in other high-intensity situations: crying at funerals, winning extravagant prizes on reality TV shows, getting their nipples and ears pierced, and having orgasms.
Again, without body language to provide context, viewers struggled to correctly read facial expressions. In fact, they rated isolated faces displaying positive emotions more negatively than faces displaying negative emotions.
Dr David Matsumoto, a psychologist at San Francisco State University in California, has his doubts about Aviezer's techniques for classifying emotion, however. He says his own research suggests that the face made by athletes when they win is a signal of competitive dominance - not necessarily a ''positive'' emotion.
Aviezer says the results could potentially help people who have difficulty recognising facial expressions. ''Maybe we should zoom out from isolated faces when we teach people how to read emotion.''
First, look at what's happening in the surroundings, he says, ''then look at the body - then, look at the face''.