You may think you're good at reading other people's faces and moods. But when it comes to telling the difference between a person who is suicidal and one who is merely depressed, the signs are much more subtle. To tease out the differences, the researchers looked at a handful of facial gestures displayed by three groups of people – those with suicidal ideation, depressed patients, and a medical control group. During interviews with these groups, they recorded gestures that included smiling, frowning, eye brow raising, and head motioning. The data was then fed into a machine-learning algorithm that looked for correlations between different gestures, alone or in combination, and patient groups.