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the first two years

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Adults don’t change much in a year or two. They might have longer, grayer, or thinner hair; they might gain or lose weight; they might learn something new. But if you saw friends you hadn’t seen for a few years, you’d recognize them immediately.

Imagine caring for a newborn 24/7 for a month and then leaving for two years. On your return, you might not recognize him or her. The baby would have quadrupled in weight, grown a foot taller, and sprouted a new head of hair. Behavior and emotions change, too—less crying, but new laughter and fear—including fear of you.

A year or two is not much compared with the 80 or so years of the average life. However, in their first two years, humans reach half their adult height, learn to talk in sentences, and express almost every emotion—not just joy and fear but also love, jealousy, and shame. Invisible changes in the brain are even more crucial, setting the pattern for the life span. The next three chapters describe these radical and awesome changes.