How often and where you use your smartphone isn't always about convenience—it may actually be a sign of undiscovered depression
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Your phone knows a lot about you: Not only can it uncover your weakness for online shoe shopping and your addiction to Candy Crush, but it can also read your pulse, track your sleep habits, motivate you to workout, and chart your period. And soon you may be able to add "monitor your mental health" to the list.

According to a small study from Northwestern University, how and where we use our phones can be a sign of depression. Researchers looked at how often participants used their phones during the day and discovered that on a daily basis, depressed people reach for their cells more than twice as often as non-depressed people do. That may seem backwards-after all, depressed people often shut themselves off from the rest of the world. And while the research team didn't know exactly what people were doing on their phones, they suspect the depressed participants weren't talking to friends or family but rather surfing the web and playing games. (This is Your Brain On: Depression.)

"People are likely, when on their phones, to avoid thinking about things that are troubling, painful feelings, or difficult relationships," said senior author David Mohr, Ph.D., a clinical psychologist and director of the Center for Behavioral Intervention Technologies at Northwestern University. "It's an avoidance behavior we see in depression."

Mohr and his colleagues also used the phones' GPS features to track the subjects' movements throughout the day, looking at how many different places they visited, where they spent the most time, and how regular their routine was. His team found that depressed subjects went less places, had inconsistent routines, and spent more time at home. (Hear one woman's victorious story: "Running Helped Me Overcome Depression and Anxiety".) "When people are depressed, they tend to withdraw and don't have the motivation or energy to go out and do things," Mohr explained.

But perhaps the most interesting part of the study was that when the phone data was compared to the results of a traditional depression screening self-questionnaire, the scientists found that the phone better predicted whether or not the person was depressed, identifying the mental illness with 86 percent accuracy.

"The significance of this is we can detect if a person has depressive symptoms and the severity of those symptoms without asking them any questions," Mohr said. "We now have an objective measure of behavior related to depression. And we're detecting it passively. Phones can provide data unobtrusively and with no effort on the part of the user." (Here, 8 Alternative Mental Health Therapies, Explained.)

The study is small and it isn't clear exactly how the link works-for example, do depressed people use their phones more or does chronic phone use make people depressed, as has been theorized in other research? But despite the limitations, researchers think this could be a huge help to both doctors and sufferers of depression, the most common mental illness. Not only could doctors identify when people are becoming depressed easier but they could use the phone data to help guide the treatment plan, whether that is encouraging the person to get out more or use their phone less.

This feature isn't available on phones (yet!), but, in the meantime, you can be your own scientist. Consider what you use your phone for most-connecting with others or retreating from the world. If it's the latter, consider talking to your doctor about your mental health and he or she can help you make smart choices with or without your smartphone.