Here we need to differentiate between population science and individual care. On a population level it is clear that teenage girls have high rates of transient (short-term) mood changes. Whether an individual girl has crossed the boundary between normal behavior and depressed behavior requires a personal evaluation with a clinician who is an expert in adolescent mental health. A family history of depression should make that consultation more urgent, whether or not the young woman’s symptoms have impaired her schoolwork, friendships, and/or family relationships. If she is not willing to go with you to see someone, seek advice about her behavior yourself with someone who can evaluate the seriousness of her symptoms.