If you could change one thing in your life to become a happier person — like your income, a job, your relationships or your health — what would make the biggest difference? That's the question Harvard psychiatrist Dr. Robert Waldinger has been attempting to answer through decades of research. He's the director of "the world's longest-running scientific study of happiness," and he spoke with Ari Shapiro about the factor that appears to make the biggest difference in people's lives. Waldinger is a co-author of The Good Life: Lessons from the world's longest scientific study of happiness.In participating regions, you'll also hear a local news segment to help you make sense of what's going on in your community.Email us at considerthis@npr.org.