Quiet (18 page)

Many of the children turned out exactly as Kagan had expected. The high-reactive infants, the 20 percent who'd hollered at the mobiles bobbing above their heads, were more likely to have developed serious, careful personalities. The low-reactive infants—the quiet ones—were more likely to have become relaxed and confident types. High and low reactivity tended to correspond, in other words, to introversion and extroversion. As Kagan mused in his 1998 book,
Galen's Prophecy
, “
Carl Jung's descriptions of the introvert and extrovert, written over seventy-five years ago, apply with uncanny accuracy to a proportion of our high- and low-reactive adolescents.”

Kagan describes two of those adolescents—
reserved Tom and extroverted Ralph—and the differences between the two are striking. Tom, who was unusually shy as a child, is good at school, watchful and quiet, devoted to his girlfriend and parents, prone to worry, and loves learning on his own and thinking about intellectual problems. He plans to be a scientist. “Like … other famous introverts who were shy children,” writes
Kagan, comparing Tom to T. S. Eliot and the mathematician-philosopher Alfred North Whitehead, Tom “has chosen a life of the mind.”

Ralph, in contrast, is relaxed and self-assured. He engages the interviewer from Kagan's team as a peer, not as an authority figure twenty-five years his senior. Though Ralph is very bright, he recently failed his English and science classes because he'd been goofing around. But nothing much bothers Ralph. He admits his flaws cheerfully.

Psychologists often discuss the difference between “temperament” and “personality.” Temperament refers to inborn, biologically based behavioral and emotional patterns that are observable in infancy and early childhood; personality is the complex brew that emerges after cultural influence and personal experience are thrown into the mix.
Some say that temperament is the foundation, and personality is the building. Kagan's work helped link certain infant temperaments with adolescent personality styles like those of Tom and Ralph.

But how did Kagan know that the arm-thrashing infants would likely turn into cautious, reflective teens like Tom, or that the quiet babies were more likely to become forthright, too-cool-for-school Ralphs? The answer lies in their physiologies.

In addition to observing the children's behaviors in strange situations, Kagan's team measured their heart rates, blood pressure, finger temperature, and other properties of the nervous system. Kagan chose these measures because they're believed to be controlled by a
potent organ inside the brain called the amygdala. The amygdala is located deep in the limbic system, an ancient brain network found even in primitive animals like mice and rats. This network—sometimes called the “emotional brain”—underlies many of the basic instincts we share with these animals, such as appetite, sex drive, and fear.

The amygdala serves as the brain's emotional switchboard, receiving information from the senses and then signaling the rest of the brain and nervous system how to respond. One of its functions is to instantly detect
new or threatening things in the environment—from an airborne Frisbee to a hissing serpent—and send rapid-fire signals through the body that trigger the fight-or-flight response.
When the Frisbee looks like it's headed straight for your nose, it's your amygdala that tells you to duck. When the rattlesnake prepares to bite, it's the amygdala that makes sure you run.

Kagan hypothesized that infants born with an especially excitable amygdala would wiggle and howl when shown unfamiliar objects—and grow up to be children who were more likely to feel vigilant when meeting new people. And this is just what he found. In other words, the four-month-olds who thrashed their arms like punk rockers did so not because they were extroverts in the making, but because their little bodies reacted strongly—they were “high-reactive”—to new sights, sounds, and smells. The quiet infants were silent not because they were future introverts—just the opposite—but because they had nervous systems that were unmoved by novelty.

The more reactive a child's amygdala, the higher his heart rate is likely to be, the more widely dilated his eyes, the tighter his vocal cords, the more cortisol (a stress hormone) in his saliva—the more jangled he's likely to
feel
when he confronts something new and stimulating. As high-reactive infants grow up, they continue to confront the unknown in many different contexts, from visiting an amusement park for the first time to meeting new classmates on the first day of kindergarten. We tend to notice most a child's reaction to unfamiliar people—how does he behave on the first day of school? Does she seem uncertain at birthday parties full of kids she doesn't know? But what we're really observing is a child's sensitivity to novelty in general, not just to people.

High- and low-reactivity are probably not the only biological routes to introversion and extroversion. There are plenty of introverts who do not have the sensitivity of a classic high-reactive, and a small percentage of high-reactives grow up to be extroverts. Still, Kagan's decades-long series of discoveries mark a dramatic breakthrough in our understanding of these personality styles—including the value judgments we make. Extroverts are sometimes credited with being “pro-social”—meaning caring about others—and introverts disparaged as people who don't like people. But the reactions of the infants in Kagan's tests had nothing
to do with people. These babies were shouting (or not shouting) over Q-tips. They were pumping their limbs (or staying calm) in response to popping balloons. The high-reactive babies were not misanthropes in the making; they were simply sensitive to their environments.

Indeed, the sensitivity of these children's nervous systems seems to be linked not only to noticing scary things, but to noticing in general. High-reactive children pay what one psychologist calls “
alert attention” to people and things.
They literally use more eye movements than others to compare choices before making a decision. It's as if they process more deeply—sometimes consciously, sometimes not—the information they take in about the world. In one early series of studies, Kagan asked a group of first-graders to play a visual matching game. Each child was shown a picture of a teddy bear sitting on a chair, alongside six other similar pictures, only one of which was an exact match. The high-reactive children spent more time than others considering all the alternatives, and were more likely to make the right choice. When Kagan asked these same kids to play word games, he found that they also read more accurately than impulsive children did.

High-reactive kids also tend to think and feel deeply about what they've noticed, and to bring an extra degree of nuance to everyday experiences. This can be expressed in many different ways. If the child is socially oriented, she may spend a lot of time pondering her observations of others—why Jason didn't want to share his toys today, why Mary got so mad at Nicholas when he bumped into her accidentally. If he has a particular interest—in solving puzzles, making art, building sand castles—he'll often concentrate with unusual intensity.
If a high-reactive toddler breaks another child's toy by mistake, studies show, she often experiences a more intense mix of guilt and sorrow than a lower-reactive child would. All kids notice their environments and feel emotions, of course, but high-reactive kids seem to see and feel things more. If you ask a high-reactive seven-year-old
how a group of kids should share a coveted toy, writes the science journalist Winifred Gallagher, he'll tend to come up with sophisticated strategies like “Alphabetize their last names, and let the person closest to A go first.”

“Putting theory into practice is hard for them,” writes Gallagher, “because their sensitive natures and elaborate schemes are unsuited
to the heterogeneous rigors of the schoolyard.” Yet as we'll see in the chapters to come, these traits—alertness, sensitivity to nuance, complex emotionality—turn out to be highly underrated powers.

Kagan has given us painstakingly documented evidence that high reactivity is one biological basis of introversion (we'll explore another likely route in
chapter 7
), but his findings are powerful in part because they confirm what we've sensed all along. Some of Kagan's studies even venture into the realm of cultural myth. For example, he believes, based on his data, that high reactivity is associated with physical traits such as
blue eyes, allergies, and hay fever, and that high-reactive men are more likely than others to have a thin body and narrow face. Such conclusions are speculative and call to mind the nineteenth-century practice of divining a man's soul from the shape of his skull. But whether or not they turn out to be accurate, it's interesting that these are just the physical characteristics we give fictional characters when we want to suggest that they're quiet, introverted, cerebral. It's as if these physiological tendencies are buried deep in our cultural unconscious.

Take Disney movies, for example: Kagan and his colleagues speculate that Disney animators unconsciously understood high reactivity when they drew sensitive figures like Cinderella, Pinocchio, and Dopey with blue eyes, and brasher characters like Cinderella's stepsisters, Grumpy, and Peter Pan with darker eyes. In many books, Hollywood films, and TV shows, too, the stock character of a reedy, nose-blowing young man is shorthand for the hapless but thoughtful kid who gets good grades, is a bit overwhelmed by the social whirl, and is talented at introspective activities like poetry or astrophysics. (Think Ethan Hawke in
Dead Poets Society
.) Kagan even speculates that some men prefer women with fair skin and blue eyes because they unconsciously code them as sensitive.

Other studies of personality also support the premise that
extroversion and introversion are physiologically, even genetically, based. One of the most common ways of untangling nature from nurture is to compare
the personality traits of identical and fraternal twins. Identical twins develop from a single fertilized egg and therefore have exactly the same genes, while fraternal twins come from separate eggs and share only 50 percent of their genes on average. So if you measure introversion or extroversion levels in pairs of twins and find more correlation in identical twins than in fraternal pairs—which scientists do, in study after study, even of twins raised in separate households—you can reasonably conclude that the trait has some genetic basis.

None of these studies is perfect, but the results have consistently suggested that introversion and extroversion, like other major personality traits such as agreeableness and conscientiousness, are about
40 to 50 percent heritable.

But are biological explanations for introversion wholly satisfying? When I first read Kagan's book
Galen's Prophecy
, I was so excited that I couldn't sleep. Here, inside these pages, were my friends, my family, myself—all of humanity, in fact!—neatly sorted through the prism of a quiescent nervous system versus a reactive one. It was as if centuries of philosophical inquiry into the mystery of human personality had led to this shining moment of scientific clarity. There was an easy answer to the nature-nurture question after all—we are born with prepackaged temperaments that powerfully shape our adult personalities.

But it couldn't be that simple—could it? Can we really reduce an introverted or extroverted personality to the nervous system its owner was born with? I would guess that I inherited a high-reactive nervous system, but my mother insists I was an easy baby, not the kind to kick and wail over a popped balloon. I'm prone to wild flights of self-doubt, but I also have a deep well of courage in my own convictions. I feel horribly uncomfortable on my first day in a foreign city, but I love to travel. I was shy as a child, but have outgrown the worst of it. Furthermore, I don't think these contradictions are so unusual; many people have dissonant aspects to their personalities. And people change profoundly over time, don't they? What about free will—do we have no control over who we are, and whom we become?