A GRIM expression in a yearbook photo or family snapshot could mean more than just a passing bad mood. It could also signal that the subject is more likely to get divorced than someone with a big smile for the camera. Matthew Hertenstein and his colleagues at DePauw University in Greencastle, Indiana asked old boys and girls of the university to answer questions about their current sexual relationships and whether they had ever been divorced. The team then looked up pictures of their volunteers in the university's yearbooks and graded the degree of their smiles. The less a person smiled, it turned out, the more likely he or she was to have been divorced over the course of a lifetime.

This research is a dramatic example of how “thin slices” of information can predict important aspects of people's personalities. In past studies, researchers have shown that with very limited information—less than half a minute of interaction, the viewing of a video clip or just a look at a photograph—people can make accurate predictions about others' sexual orientation, socioeconomic status, teaching ability and personality.

Dr Hertenstein was following up research which had shown that the women who smiled most in their college photos were most likely to be married by the age of 27, among other things. He wanted to see if the same held, over the longer term, for divorce. His study, to be published in Motivation & Emotion, looked at three groups. The first, of 306 people, came from alumni of the psychology department. The second group, of 349, was recruited from general alumni. The third, of 55 people, was recruited from the town. (In the last case, people were asked to send in photos of themselves, but were not told that the study was about smiling.) The researchers rated the photos of the subjects on a scale of two to ten. They also asked their volunteers various questions, including whether they had ever been divorced.

The relationship between smiling and divorce also held up among townspeople, even though many sent photographs of themselves as children. Facial expression predicted divorce even when the smile or frown was on a ten-year-old's face. A photograph that records a split second from a lifetime is a very thin slice indeed. How could it predict a divorce decades in the future?

The researchers suggest that the smiles are accurate indicators of personality. The results should not be overstretched. The never-divorced had their smiles rated on average at 5.9, 5.9 and 5.2 out of 10 in each of the three groups, while the divorced scored 5, 5.3 and 4.4. That is not a huge difference, but it is statistically significant. On the other hand, comparing only the lowest-scoring people with the highest-scoring, the least-smiling were three times more likely than the biggest smilers to divorce.

Until the findings are replicated it is probably too early to choose a spouse based on a facial expression in a photo. On the other hand, it would not hurt to smile for the camera yourself.