Jerome Kagan: How Babies Become Introverts

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Who is Jerome Kagan? How did his longitudinal research on babies show how introversion and extroversion can be identified at an early age?

Jerome Kagan is a developmental psychologist that started a multi-year study in 1989. That study showed that reactivity as an infant was a predictor of temperament later in life.

Keep reading to find out more about Jerome Kagan and his research.

Jerome Kagan: High Reactives and Introversion

In 1989, developmental psychologist Jerome Kagan of Harvard began a longitudinal or years-long study of 500 four-month-old babies and was able to tell which ones would turn out to be extroverts and which would be introverts based on innate temperament. 

By introducing stimuli such as voices, popping balloons, or smells, Kagan determined which infants were high reactives (crying and throwing their arms around) and which were low-reactive. Counterintuitively, he predicted the highly reactive babies were likely to be quiet or introverts as teenagers, which turned out to be the case. 

He did further testing of their reactions to new things—for instance, a clown and a woman wearing a gas mask—when the children were two, four, seven, and eleven. Most of the children turned out exactly as Kagan expected: high-reactive children turned out to be introverts while low-reactive children were extroverts.

Alertness and Sensitivity

Jerome Kagan also found that highly reactive children are more sensitive to their environment.

Besides noting the children’s behavioral reactions to strange situations, Jerome Kagan measured their heart rate, blood pressure, and finger temperature, which are controlled by the amygdala region—or emotional control center—of the brain. One function of the amygdala is to detect new or threatening things and trigger a reaction, such as ducking when a ball comes at your head. He found that highly reactive infants’ nervous systems reacted strongly to unfamiliar things while quiet infants’ nervous systems were unaffected.

As highly reactive children grow, parents and teachers may notice a wary reaction to unfamiliar people or the first day of preschool and may assume a child is shy. But what they’re really reacting to is newness. High reactives are just more sensitive to their environment. One psychologist calls this “alert attention”—they notice not only alarming things, but also most things in general. 


High-reactive children also tend to more thoroughly process what they’ve observed and to apply more nuance to everyday experiences. For instance, they may spend a lot of time thinking about others’ actions, considering potential reasons why another child did what he did.

Jerome Kagan found that highly reactive kids seem to feel emotions more strongly as well—for instance, they feel more intense guilt if they break something than low-reactive children feel. They’ll also concentrate more intensely on something that interests them.

Jerome Kagan: How Babies Become Introverts

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Here's what you'll find in our full Quiet: The Power of Introverts summary :

  • How society overvalues extroverts
  • Why introverts' overlooked strengths are the key to greater success in work, school, and society
  • How extroversion caused the fall of Enron

Rina Shah

An avid reader for as long as she can remember, Rina’s love for books began with The Boxcar Children. Her penchant for always having a book nearby has never faded, though her reading tastes have since evolved. Rina reads around 100 books every year, with a fairly even split between fiction and non-fiction. Her favorite genres are memoirs, public health, and locked room mysteries. As an attorney, Rina can’t help analyzing and deconstructing arguments in any book she reads.

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