I'm not normally the suicidal type, but this is the night before a big speech, and my mind races with horrifying what-if propositions. What if my mouth dries up and I can't get any words out? What if I bore the audience? What if I throw up onstage?

My boyfriend (now my husband), Ken, watches me toss and turn. He's bewildered by my distress. A former UN peacekeeper, he once was ambushed in Somalia, yet I don't think he felt as scared then as I do now.

"Try to think of happy things," he says, caressing my forehead.

I stare at the ceiling, tears welling. What happy things? Who could be happy in a world of podiums and microphones?

"There are a billion people in China who don't give a rat's a.s.s about your speech," Ken offers sympathetically.

This helps, for approximately five seconds. I turn over and watch the alarm clock. Finally it's six thirty. At least the worst part, the night-before part, is over; this time tomorrow, I'll be free. But first I have to get through today. I dress grimly and put on a coat. Ken hands me a sports water bottle filled with Baileys Irish Cream. I'm not a big drinker, but I like Baileys because it tastes like a chocolate milkshake. "Drink this fifteen minutes before you go on," he says, kissing me good-bye.

I take the elevator downstairs and settle into the car that waits to ferry me to my destination, a big corporate headquarters in suburban New Jersey. The drive gives me plenty of time to wonder how I allowed myself to get into this situation. I recently left my job as a Wall Street lawyer to start my own consulting firm. Mostly I've worked one-on-one or in small groups, which feels comfortable. But when an acquaintance who is general counsel at a big media company asked me to run a seminar for his entire executive team, I agreed-enthusiastically, even!-for reasons I can't fathom now. I find myself praying for calamity-a flood or a small earthquake, maybe-anything so I don't have to go through with this. Then I feel guilty for involving the rest of the city in my drama.

The car pulls up at the client's office and I step out, trying to project the peppy self-a.s.surance of a successful consultant. The event organizer escorts me to the auditorium. I ask for directions to the bathroom, and, in the privacy of the stall, gulp from the water bottle. For a few moments I stand still, waiting for the alcohol to work its magic. But nothing happens-I'm still terrified. Maybe I should take another swig. No, it's only nine in the morning-what if they smell the liquor on my breath? I reapply my lipstick and make my way back to the event room, where I arrange my notecards at the podium as the room fills with important-looking businesspeople. Whatever you do, try not to vomit, I tell myself.

Some of the executives glance up at me, but most of them stare fixedly at their BlackBerrys. Clearly, I'm taking them away from very pressing work. How am I going to hold their attention long enough for them to stop pounding out urgent communiques into their tiny typewriters? I vow, right then and there, that I will never make another speech.

Well, since then I've given plenty of them. I haven't completely overcome my anxiety, but over the years I've discovered strategies that can help anyone with stage fright who needs to speak in public. More about that in chapter 5.

In the meantime, I've told you my tale of abject terror because it lies at the heart of some of my most urgent questions about introversion. On some deep level, my fear of public speaking seems connected to other aspects of my personality that I appreciate, especially my love of all things gentle and cerebral. This strikes me as a not-uncommon constellation of traits. But are they truly connected, and if so, how? Are they the result of "nurture"-the way I was raised? Both of my parents are soft-spoken, reflective types; my mother hates public speaking too. Or are they my "nature"-something deep in my genetic makeup?

I've been puzzling over these questions for my entire adult life. Fortunately, so have researchers at Harvard, where scientists are probing the human brain in an attempt to discover the biological origins of human temperament.

One such scientist is an eighty-two-year-old man named Jerome Kagan, one of the great developmental psychologists of the twentieth century. Kagan devoted his career to studying the emotional and cognitive development of children. In a series of groundbreaking longitudinal studies, he followed children from infancy through adolescence, doc.u.menting their physiologies and personalities along the way. Longitudinal studies like these are time-consuming, expensive, and therefore rare-but when they pay off, as Kagan's did, they pay off big.

For one of those studies, launched in 1989 and still ongoing, Professor Kagan and his team gathered five hundred four-month-old infants in his Laboratory for Child Development at Harvard, predicting they'd be able to tell, on the strength of a forty-five-minute evaluation, which babies were more likely to turn into introverts or extroverts. If you've seen a four-month-old baby lately, this may seem an audacious claim. But Kagan had been studying temperament for a long time, and he had a theory.

Kagan and his team exposed the four-month-olds to a carefully chosen set of new experiences. The infants heard tape-recorded voices and balloons popping, saw colorful mobiles dance before their eyes, and inhaled the scent of alcohol on cotton swabs. They had wildly varying reactions to the new stimuli. About 20 percent cried l.u.s.tily and pumped their arms and legs. Kagan called this group "high-reactive." About 40 percent stayed quiet and placid, moving their arms or legs occasionally, but without all the dramatic limb-pumping. This group Kagan called "low-reactive." The remaining 40 percent fell between these two extremes. In a startlingly counterintuitive hypothesis, Kagan predicted that it was the infants in the high-reactive group-the l.u.s.ty arm-pumpers-who were most likely to grow into quiet teenagers.

When they were two, four, seven, and eleven years old, many of the children returned to Kagan's lab for follow-up testing of their reactions to new people and events. At the age of two, the children met a lady wearing a gas mask and a lab coat, a man dressed in a clown costume, and a radio-controlled robot. At seven, they were asked to play with kids they'd never met before. At eleven, an unfamiliar adult interviewed them about their personal lives. Kagan's team observed how the children reacted to these strange situations, noting their body language and recording how often and spontaneously they laughed, talked, and smiled. They also interviewed the kids and their parents about what the children were like outside the laboratory. Did they prefer one or two close friends to a merry band? Did they like visiting new places? Were they risk-takers or were they more cautious? Did they consider themselves shy or bold?

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, p.r.o.ne 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-a.s.sured. 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 cla.s.ses 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 appet.i.te, s.e.x 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 amus.e.m.e.nt park for the first time to meeting new cla.s.smates 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 cla.s.sic 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 b.u.mped 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 doc.u.mented 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 a.s.sociated 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 p.r.o.ne 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?

I decided to track down Professor Kagan to ask him these questions in person. I felt drawn to him not only because his research findings were so compelling, but also because of what he represents in the great nature-nurture debate. He'd launched his career in 1954 staunchly on the side of nurture, a view in step with the scientific establishment of the day. Back then, the idea of inborn temperament was political dynamite, evoking the specter of n.a.z.i eugenics and white supremacism. By contrast, the notion of children as blank slates for whom anything was possible appealed to a nation built on democracy.

But Kagan had changed his mind along the way. "I have been dragged, kicking and screaming, by my data," he says now, "to acknowledge that temperament is more powerful than I thought and wish to believe." The publication of his early findings on high-reactive children in Science magazine in 1988 helped to legitimize the idea of inborn temperament, partly because his "nurturist" reputation was so strong.

If anyone could help me untangle the nature-nurture question, I hoped, it was Jerry Kagan.

Kagan ushers me inside his office in Harvard's William James Hall, surveying me unblinkingly as I sit down: not unkind, but definitely discerning. I had imagined him as a gentle, white-lab-coated scientist in a cartoon, pouring chemicals from one test tube to another until-poof! Now, Susan, you know exactly who you are. But this isn't the mild-mannered old professor I'd imagined. Ironically for a scientist whose books are infused with humanism and who describes himself as having been an anxious, easily frightened boy, I find him downright intimidating. I kick off our interview by asking a background question whose premise he disagrees with. "No, no, no!" he thunders, as if I weren't sitting just across from him.

The high-reactive side of my personality kicks into full gear. I'm always soft-spoken, but now I have to force my voice to come out louder than a whisper (on the tape recording of our conversation, Kagan's voice sounds booming and declamatory, mine much quieter). I'm aware that I'm holding my torso tensely, one of the telltale signs of the high-reactive. It feels strange to know that Kagan must be observing this too-he says as much, nodding at me as he notes that many high-reactives become writers or pick other intellectual vocations where "you're in charge: you close the door, pull down the shades and do your work. You're protected from encountering unexpected things." (Those from less educated backgrounds tend to become file clerks and truck drivers, he says, for the same reasons.)

I mention a little girl I know who is "slow to warm up." She studies new people rather than greeting them; her family goes to the beach every weekend, but it takes her ages to dip a toe into the surf. A cla.s.sic high-reactive, I remark.

"No!" Kagan exclaims. "Every behavior has more than one cause. Don't ever forget that! For every child who's slow to warm up, yes, there will be statistically more high-reactives, but you can be slow to warm up because of how you spent the first three and a half years of your life! When writers and journalists talk, they want to see a one-to-one relationship-one behavior, one cause. But it's really important that you see, for behaviors like slow-to-warm-up, shyness, impulsivity, there are many routes to that."

He reels off examples of environmental factors that could produce an introverted personality independently of, or in concert with, a reactive nervous system: A child might enjoy having new ideas about the world, say, so she spends a lot of time inside her head. Or health problems might direct a child inward, to what's going on inside his body.

My fear of public speaking might be equally complex. Do I dread it because I'm a high-reactive introvert? Maybe not. Some high-reactives love public speaking and performing, and plenty of extroverts have stage fright; public speaking is the number-one fear in America, far more common than the fear of death. Public speaking phobia has many causes, including early childhood setbacks, that have to do with our unique personal histories, not inborn temperament.