PhotoYou can put on a brave face for family and friends, but your phone knows. It can tell when you're depressed, according to researchers at Northwestern Medicine.

By tracking the number of minutes you use your phone, the researchers claim they can get a clue about your emotional well-being. If you're a normal, well-adjusted person, you use your phone about 17 minutes a day. If you're depressed, you're on it 68 minutes a day on average.

The researchers also get a clue from tracking your phone's location. For example, if you spend most of your time in just a few locations, researchers say it increases the likelihood you're depressed. The same holds true for irregular schedules, where an individual leaves the house and goes to work at different times each day.

87% accuracy

Unlikely you say? Well, the Northwestern researchers put their theory to the test. Based on the phone sensor data which was collected, they said they could identify people with depressive symptoms with 87% 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," said senior author David Mohr. "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."

It's important, says Mohr, because it could ultimately help doctors monitor people at risk of depression and enable health care providers to intervene more quickly.

The smart phone data was actually more accurate in detecting depression than asking people about how sad they were feeling on a scale of 1 to 10. Lead author Sohrob Saeb says that when you ask people that question, their answers are unreliable and not very precise. Their phone, on the other hand, doesn't lie.

Loss of motivation

"The data showing depressed people tended not to go many places reflects the loss of motivation seen in depression," said Mohr, who is a clinical psychologist and professor of preventive medicine at Northwestern. "When people are depressed, they tend to withdraw and don't have the motivation or energy to go out and do things."

The phone data doesn't tell doctors what subjects were doing with their devices. Mohr suspects people who spent the most time on them were either surfing the web or playing games, rather than talking to friends.

"People are likely, when on their phones, to avoid thinking about things that are troubling, painful feelings or difficult relationships," he said. "It's an avoidance behavior we see in depression."

To arrive at the team's conclusions, Saeb analyzed the GPS locations and phone usage for 28 individuals, average age 29, over two weeks. The sensor tracked GPS locations every five minutes.

Half the participants had been diagnosed with some level of depression – half had not. Saeb developed algorithms using the GPS and phone usage data collected from the phone, and correlated the results of those GPS and phone usage algorithms with the subjects' depression test results. The results matched up in 87% of cases.

Though privacy issues would need to be addressed, the findings may one day be used to monitor people who are at risk of depression and offer them interventions if the sensor detected depression. It could also be used to deliver the information to their clinicians.

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