In the 1980s, when data from the world’s longest-running study on happiness started to show that good relationships kept us healthier and happier, the researchers didn’t really believe it. “We know there’s a mind-body connection and we all pay lip service to it,” says Dr Robert Waldinger, the director of the Harvard Study of Adult Development, which has been running for 84 years. “But how could warmer relationships make it less likely that you would develop coronary artery disease or arthritis? How could relationships get into the body and affect our physiology?” Then, other studies started to show the same....