In the world of science, a longitudinal study is a research method in which the same group of subjects is observed and measured over a period of time. If there is one study that really puts the "long" in longitudinal, it's the Harvard Study of Adult Development. It has been providing data on the same group of men since 1938. There were 268 of them then – fewer than 20 are still alive – all Harvard sophomores, including future President John F. Kennedy and future Washington Post editor Ben Bradlee. The goal of the study was to find out what factors lead to healthy and happy lives. And perhaps the biggest key to well-being, it has revealed, is having friends.

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