Smartphones are perhaps the most significant technological advancement in human history. From a $600, pocket-sized computer, people can communicate via email with friends across the globe, change the channels on their television and look at copious amounts of pornography. As it turns out, there’s something else your smartphone can do: It can tell researchers if you’re depressed.

Using the sensor data captured by smartphones, scientists at Northwestern University were able to determine whether someone displayed depressive symptoms with 87% accuracy. The more someone uses their phone and the fewer places they go, the more likely they are to be depressed.

“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,” said senior author David Mohr, director of the Center for Behavioral Intervention Technologies at Northwestern University Feinberg School of Medicine. “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.”

The average daily usage for depressed individuals was about 68 minutes, while for non-depressed individuals it was about 17 minutes.Moreover, not only do depressed individuals not go as many places as the non-depressed, but their schedules are less predictable as well.

Phone data is particularly useful because it draws out two major components of depression: Reluctance to tackle life’s issues, and lack of motivation. Someone who spends an hour or more each day on their phone may be using it as a form of escapism, a way to avoid confronting difficult issues or relationships. The researchers don’t know exactly how people use their smartphones, but they suspect that in depressed individuals, most of their time is spent surfing the web or playing games, rather than communicating with friends.

Similarly, the GPS data suggests that depressed people tend to stay in one place because their condition saps them of the motivation and energy required to get out into the world. Depressed people tend to withdraw from others, and the convenience of a smartphone means they may have little reason to leave the house.

In the study of 28 people (half of whom displayed no signs of depression in an initial evaluation), the smartphone data proved to be more accurate at identifying depression than daily questions the participants answered about how sad they were feeling on a scale of 1 to 10.

In the future, smartphones themselves may be able to identify the warning signs of depression, and either communicate them to the user or even physicians. The researchers also wonder whether getting people to change the behavior associated with depression could help ward off the condition itself.