Living through weddings or divorces, job losses and children’s triumphs, we sometimes feel better and sometimes feel worse. But, psychologists observe, we tend to drift back to a “set point” — a stable resting point, or baseline, in the mind’s level of contentment or unease. Research has shown that the set points for depression and anxiety are particularly stable over time. Why? “The overwhelming view within psychiatry and psychology is that is due to genetic factors,” says Virginia Commonwealth University psychiatrist Kenneth S. Kendler. “Yet we know that extreme environmental adversities,...