Could the devices being blamed for teen depression be useful in revealing it?

Studies have linked heavy smartphone use with worsening teen mental health. But as teens spend time on sites like Instagram, Snapchat and YouTube, they also leave digital trails that may offer signs about their mental well-being.

Experts say possible warning signs include changes in writing speed, voice quality, word choice and how often a student stays home from school.

There are more than 1,000 smartphone "biomarkers," said Dr. Thomas Insel, former head of the National Institute of Mental Health, which is the largest mental health research organization in the world. Insel is a leader in the smartphone psychiatry movement.

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Researchers are testing smartphone apps that use artificial intelligence, or AI, to predict depression and possible self-harm. Using smartphones as mental health detectors require permission from users to download an app, and permission could be revoked any time.

Nick Allen, a psychologist at the University of Oregon, has created an app being tested on young people who have attempted suicide. Allen says the biggest barrier is discerning the mental health crisis signals in the information on people's phones.

Suicide is the second leading cause of death for people between the ages of 10 and 34 in the United States. By 2015, suicide rates among teen boys rose to 14 in every 100,000 and five in every 100,000 people, among girls. A recent study suggested a rise in smartphone use has probably worsened the crisis.

People with mental illness, Insel said, usually get treatment "when they're in crisis and very late. ... We want to have a method to identify the earliest signs."

If smartphones can become effective predictors, app developers say the goal might be to offer automated text messages and links to assistance, or digital messages to parents, doctors and first responders.

Facebook employs "proactive detection." Last year, after a suicide was broadcast on Facebook Live, the company trained its AI systems to look for words in online posts that could predict possible self-harm. Friends' comments expressing concern about the user's well-being are part of that detection system.

Facebook has helped first responders quickly reach around 3,500 people in the past year. But the company did not offer followup details on those people.

Ongoing research includes a Stanford University study of about 200 teens. Many of them are at risk for depression because of bullying, family issues or other problems. Teens who have been studied since grade school get an experimental phone app that asks them questions about their mood three times a day for two weeks.

Laurel Foster, 15, is part of the study. Foster said she is stressed about school and friendships. Depression is common at her San Francisco high school, she said. The smartphone app felt a little like being spied on, she said, but many websites are already following users' behaviors.

Alyssa Lizarraga, 19, is also part of the study. Lizarraga said she has had depression since high school, and worries about her heavy use of smartphones and social media. She said comparing herself with others online sometimes causes her sadness. But she believes using smartphones to identify mental health problems might help push people to seek early treatment.

At the University of California, Los Angeles, researchers offer online counseling and an experimental phone app to students who show signs of at least minor depression on a test. It is part of a larger effort launched in 2017 by the university to battle depression in its students. About 250 UCLA students agreed to use the app during their first year.

At the University of Illinois' Chicago campus, researchers are using crowdsourcing to test their experimental phone app. Nearly 2,000 people have downloaded the app and agreed to let researchers follow typing behaviors. Alex Leow, a professor of psychiatry and bioengineering at the university, helped develop the app.

The study is for people 18 and older, but Leow said it could also be used for children if successful.

Along with studies at universities, technology companies such as Mindstrong and Verily — the tech health division of Google — are testing their own experimental apps.

The Associated Press contributed to this report.