When it comes to understanding what happens to people as they go through life, pictures of entire lives—of the choices people make and the paths they follow, and how it all works out for them—are almost impossible to get. Most of what we know about human life we know from asking people to remember the past, and memories are full of holes. The more time that passes, the more details we forget, and research shows that the act of recalling an event can actually change our memory of it. But what if we could watch entire lives as they unfold through time? What if we could study people from the time that they were teenagers all the way into old age to see what really matters to a person’s health and happiness, and which investments really paid off?

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