Aristotle thought the face was a window onto a person's mind. Cicero agreed. Two thousand years passed, and facial expressions are still commonly thought to be a universally valid way to judge other people's feelings, irrespective of age, sex and culture. A raised eyebrow suggests confusion. A smile indicates happiness.
Or do they? An analysis of hundreds of research papers that examined the relationship between facial expressions and underlying emotions has uncovered a surprising conclusion: there is no good scientific evidence to suggest that there are such things as recognizable facial expressions for basic emotions which are universal across cultures. Just because a person is not smiling, the researchers found, does not mean that person is unhappy.
This may raise questions about the efforts of information-technology companies to develop artificial-intelligence algorithms (算法) which can recognize facial expressions and work out a person's underlying emotional state. Microsoft, for example, claims its "Emotion API" is able to detect what people are feeling by examining video footage of them. Another of the study's authors, however, expressed scepticism. Aleix Martinez, a computer engineer at Ohio State University, said that companies attempting to obtain emotions from images of faces have failed to understand the importance of context.
For a start, facial expression is but one of a number of non-verbal ways，such as body posture, that people use to communicate with each other. Machine recognition of emotion needs to take account of these as well. But context can reach further than that. Dr Martinez mentioned an experiment in which participants were shown a close-up picture of a man's face, which was bright red with his mouth open in a scream. Based on this alone, most participants said the man was extremely angry. Then the whole picture was shown. It was a football player with his arms outstretched, celebrating a goal. His angry-looking face was, in fact, a show of pure joy.
Given that people cannot guess each other's emotional states most of the time, Dr Martinez sees no reason computers would be able to. "There are companies right now claiming to be able to do that and apply this to places I find really scary and dangerous, for example, in hiring people," he says. "Some companies require you to present a video resume, which is analyzed by a machine-learning system. And depending on your facial expressions, they hire you or not, which I find really shocking."
