Jerome Kagan’s “Aha!” moment came with Baby 19. It was 1989, and Kagan, a professor of psychology at Harvard, had just begun a major longitudinal study of temperament and its effects.

Temperament is a complex, multilayered thing and Kagan was tracking it along a single dimension: whether babies were easily upset when exposed to new things.

It seemed to explain much of normal human variation. Extrapolating from a study he had completed on toddlers, he suspected that the most edgy infants were more likely to grow up to be inhibited, shy and anxious.

He found no high-reactors among the first 18. They gazed calmly at things unfamiliar. But the 19th baby was different. She was distressed by novelty — new sounds, new voices, new toys, new smells — and showed it by flailing her legs, arching her back and crying.

Here was what Kagan, one of the most influential developmental psychologists of the 20th Century, was looking for: a baby who essentially fell apart when exposed to anything new.

Baby 19 grew up true to her temperament. A video of her from 2004, when she was 15, showed a plain-looking teenager, hiding behind her long, dark hair. The interview, given to all 15-year-olds in the longitudinal study, began with questions about school. She had very few extracurricular activities, she said, but she does like writing and playing the violin. She fidgeted constantly, twirling her hair, touching her ear, jiggling her knee.

Then interviewer asked what she worried about. “I don’t know,” Baby 19 said after a long pause, twirling her hair faster, touching her face, her knee. Then the list of troubles spilled out: “When I don’t quite know what to do and it’s really frustrating. I worry about things like getting projects done... I think, Will I get it done? How am I going to do it? ... If I’m going to be in a big crowd, that even makes me nervous. How I’m going to deal with the world when I’m grown?”

... contd.