"Morency and his colleagues are particularly interested in using machine learning to trace connections between facial expressions and emotional state among depressed people. And what they’ve found so far is unexpected. For one thing, depressed people and non-depressed people smile with the same frequency. But the kinds of smiling they did were different. So while depressed people smiled as often as non-depressed people, the depressed people’s smiles lasted for a shorter period of time. […]

There was also a pronounced gender difference in facial expressions among depressed people. In one University of Southern California study, Morency and three other researchers found that depressed men frown more often then non-depressed men, but observed the opposite effect among women: Depressed women frowned less frequently than non-depressed women."