Rising rates of depression among American teenagers and young adults have led to a major question: Could the same devices being blamed for causing depression be used to find 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 paths that may offer signs about their mental wellbeing. Experts say possible warning signs include changes in writing speed, voice quality, word choice and how often a child stays home from school. There are more than 1,000 smartphone “biomarkers,” said Dr. Thomas Insel. He is former head of the National Institute of Mental Health. He has become a leader in the smartphone psychiatry movement. Researchers are testing smartphone apps that use artificial intelligence, or AI, to predict depression and possible self-harm. But there are still issues to address, including privacy issues and making sure children give permission to be closely followed.