If you want to study happiness, you first have to define what you mean by the word. There are the momentary pleasures of a great glass of wine, a smile on a child’s face, or a soft couch after a long day. But there are also deeper meanings of the word. The ancient Greek word for happiness, eudemonia, conveys not fleeting good feelings but rather the sum total of a life well lived, including a sense of meaning and virtue. To figure out how to best achieve this higher form of happiness, you can’t ask what cheers people up or brings them down, day to day. You have to follow them over decades to determine what really makes them flourish and feel fulfilled as they approach the end of their days.