The Harvard Study of Adult Development is a decades-long project that’s resulted in a greater understanding of the importance of healthy relationships. The research began with a group of 268 sophomores at Harvard University in the 1930s and later expanded to, among others, 456 Boston inner-city residents. Researchers have conducted interviews with the participants and pored over their medical records every few years. They discovered that close relationships are the biggest factor in predicting happiness and health. It turns out that if we surround ourselves with the right people, we’ll likely experience a deeper sense of joy.