The results were unambiguous. The men in twenty-three of the twenty-four groups produced more ideas when they worked on their own than when they worked as a group. They also produced ideas of equal or higher quality when working individually. And the advertising executives were no better at group work than the presumably introverted research scientists.

Since then, some forty years of research has reached the same startling conclusion. Studies have shown that performance gets worse as group size increases: groups of nine generate fewer and poorer ideas compared to groups of six, which do worse than groups of four. The "evidence from science suggests that business people must be insane to use brainstorming groups," writes the organizational psychologist Adrian Furnham. "If you have talented and motivated people, they should be encouraged to work alone when creativity or efficiency is the highest priority."

The one exception to this is online brainstorming. Groups brainstorming electronically, when properly managed, not only do better than individuals, research shows; the larger the group, the better it performs. The same is true of academic research—professors who work together electronically, from different physical locations, tend to produce research that is more influential than those either working alone or collaborating face-to-face.

This shouldn't surprise us; as we've said, it was the curious power of electronic collaboration that contributed to the New Groupthink in the first place. What created Linux, or Wikipedia, if not a gigantic electronic brainstorming session? But we're so impressed by the power of online collaboration that we've come to overvalue all group work at the expense of solo thought. We fail to realize that participating in an online working group is a form of solitude all its own. Instead we assume that the success of online collaborations will be replicated in the face-to-face world.

Indeed, after all these years of evidence that conventional brainstorming groups don't work, they remain as popular as ever. Participants in brainstorming sessions usually believe that their group performed much better than it actually did, which points to a valuable reason for their continued popularity—group brainstorming makes people feel attached. A worthy goal, so long as we understand that social glue, as opposed to creativity, is the principal benefit.

Psychologists usually offer three explanations for the failure of group brainstorming. The first is social loafing: in a group, some individuals tend to sit back and let others do the work. The second is production blocking: only one person can talk or produce an idea at once, while the other group members are forced to sit passively. And the third is evaluation apprehension, meaning the fear of looking stupid in front of one's peers.

Osborn's "rules" of brainstorming were meant to neutralize this anxiety, but studies show that the fear of public humiliation is a potent force. During the 1988–89 basketball season, for example, two NCAA basketball teams played eleven games without any spectators, owing to a measles outbreak that led their schools to quarantine all students. Both teams played much better (higher free-throw percentages, for example) without any fans, even adoring home-team fans, to unnerve them.

The behavioral economist Dan Ariely noticed a similar phenomenon when he conducted a study asking thirty-nine participants to solve anagram puzzles, either alone at their desks or with others watching. Ariely predicted that the participants would do better in public because they'd be more motivated. But they performed worse. An audience may be rousing, but it's also stressful.

The problem with evaluation apprehension is that there's not much we can do about it. You'd think you could overcome it with will or training or a set of group process rules like Alex Osborn's. But recent research in neuroscience suggests that the fear of judgment runs much deeper and has more far-reaching implications than we ever imagined.

Between 1951 and 1956, just as Osborn was promoting the power of group brainstorming, a psychologist named Solomon Asch conducted a series of now-famous experiments on the dangers of group influence. Asch gathered student volunteers into groups and had them take a vision test. He showed them a picture of three lines of varying lengths and asked questions about how the lines compared with one another: which was longer, which one matched the length of a fourth line, and so on. His questions were so simple that 95 percent of students answered every question correctly.

But when Asch planted actors in the groups, and the actors confidently volunteered the same incorrect answer, the number of students who gave all correct answers plunged to 25 percent. That is, a staggering 75 percent of the participants went along with the group's wrong answer to at least one question.

The Asch experiments demonstrated the power of conformity at exactly the time that Osborn was trying to release us from its chains. What they didn't tell us was why we were so prone to conform. What was going on in the minds of the kowtowers? Had their perception of the lines' lengths been altered by peer pressure, or did they knowingly give wrong answers for fear of being the odd one out? For decades, psychologists puzzled over this question.

Today, with the help of brain-scanning technology, we may be getting closer to the answer. In 2005 an Emory University neuroscientist named Gregory Berns decided to conduct an updated version of Asch's experiments. Berns and his team recruited thirty-two volunteers, men and women between the ages of nineteen and forty-one. The volunteers played a game in which each group member was shown two different three-dimensional objects on a computer screen and asked to decide whether the first object could be rotated to match the second. The experimenters used an fMRI scanner to take snapshots of the volunteers' brains as they conformed to or broke with group opinion.

The results were both disturbing and illuminating. First, they corroborated Asch's findings. When the volunteers played the game on their own, they gave the wrong answer only 13.8 percent of the time. But when they played with a group whose members gave unanimously wrong answers, they agreed with the group 41 percent of the time.

But Berns's study also shed light on exactly why we're such conformists. When the volunteers played alone, the brain scans showed activity in a network of brain regions including the occipital cortex and parietal cortex, which are associated with visual and spatial perception, and in the frontal cortex, which is associated with conscious decision-making. But when they went along with their group's wrong answer, their brain activity revealed something very different.

Remember, what Asch wanted to know was whether people conformed despite knowing that the group was wrong, or whether their perceptions had been altered by the group. If the former was true, Berns and his team reasoned, then they should see more brain activity in the decision-making prefrontal cortex. That is, the brain scans would pick up the volunteers deciding consciously to abandon their own beliefs to fit in with the group. But if the brain scans showed heightened activity in regions associated with visual and spatial perception, this would suggest that the group had somehow managed to change the individual's perceptions.

That was exactly what happened—the conformists showed less brain activity in the frontal, decision-making regions and more in the areas of the brain associated with perception. Peer pressure, in other words, is not only unpleasant, but can actually change your view of a problem.

These early findings suggest that groups are like mind-altering substances. If the group thinks the answer is A, you're much more likely to believe that A is correct, too. It's not that you're saying consciously, "Hmm, I'm not sure, but they all think the answer's A, so I'll go with that." Nor are you saying, "I want them to like me, so I'll just pretend that the answer's A." No, you are doing something much more unexpected—and dangerous. Most of Berns's volunteers reported having gone along with the group because "they thought that they had arrived serendipitously at the same correct answer." They were utterly blind, in other words, to how much their peers had influenced them.

What does this have to do with social fear? Well, remember that the volunteers in the Asch and Berns studies didn't always conform. Sometimes they picked the right answer despite their peers' influence. And Berns and his team found something very interesting about these moments. They were linked to heightened activation in the amygdala, a small organ in the brain associated with upsetting emotions such as the fear of rejection.

Berns refers to this as "the pain of independence," and it has serious implications. Many of our most important civic institutions, from elections to jury trials to the very idea of majority rule, depend on dissenting voices. But when the group is literally capable of changing our perceptions, and when to stand alone is to activate primitive, powerful, and unconscious feelings of rejection, then the health of these institutions seems far more vulnerable than we think.

But of course I've been simplifying the case against face-to-face collaboration. Steve Wozniak collaborated with Steve Jobs, after all; without their pairing, there would be no Apple today. Every pair bond between mother and father, between parent and child, is an act of creative collaboration. Indeed, studies show that face-to-face interactions create trust in a way that online interactions can't. Research also suggests that population density is correlated with innovation; despite the advantages of quiet walks in the woods, people in crowded cities benefit from the web of interactions that urban life offers.

I have experienced this phenomenon personally. When I was getting ready to write this book, I carefully set up my home office, complete with uncluttered desk, file cabinets, free counter space, and plenty of natural light—and then felt too cut off from the world to type a single keystroke there. Instead, I wrote most of this book on a laptop at my favorite densely packed neighborhood café. I did this for exactly the reasons that champions of the New Groupthink might suggest: the mere presence of other people helped my mind to make associative leaps. The coffee shop was full of people bent over their own computers, and if the expressions of rapt concentration on their faces were any indication, I wasn't the only one getting a lot of work done.

But the café worked as my office because it had specific attributes that are absent from many modern schools and workplaces. It was social, yet its casual, come-and-go-as-you-please nature left me free from unwelcome entanglements and able to "deliberately practice" my writing. I could toggle back and forth between observer and social actor as much as I wanted. I could also control my environment. Each day I chose the location of my table—in the center of the room or along the perimeter—depending on whether I wanted to be seen as well as to see. And I had the option to leave whenever I wanted peace and quiet to edit what I'd written that day. Usually I was ready to exercise this right after only a few hours—not the eight, ten, or fourteen hours that many office dwellers put in.

The way forward, I'm suggesting, is not to stop collaborating face-to-face, but to refine the way we do it. For one thing, we should actively seek out symbiotic introvert-extrovert relationships, in which leadership and other tasks are divided according to people's natural strengths and temperaments. The most effective teams are composed of a healthy mix of introverts and extroverts, studies show, and so are many leadership structures.

We also need to create settings in which people are free to circulate in a shifting kaleidoscope of interactions, and to disappear into their private workspaces when they want to focus or simply be alone. Our schools should teach children the skills to work with others—cooperative learning can be effective when practiced well and in moderation—but also the time and training they need to deliberately practice on their own. It's also vital to recognize that many people—especially introverts like Steve Wozniak—need extra quiet and privacy in order to do their best work.

Some companies are starting to understand the value of silence and solitude, and are creating "flexible" open plans that offer a mix of solo workspaces, quiet zones, casual meeting areas, cafés, reading rooms, computer hubs, and even "streets" where people can chat casually with each other without interrupting others' workflow. At Pixar Animation Studios, the sixteen-acre campus is built around a football-field-sized atrium housing mailboxes, a cafeteria, and even bathrooms. The idea is to encourage as many casual, chance encounters as possible. At the same time, employees are encouraged to make their individual offices, cubicles, desks, and work areas their own and to decorate them as they wish. Similarly, at Microsoft, many employees enjoy their own private offices, yet they come with sliding doors, movable walls, and other features that allow occupants to decide when they want to collaborate and when they need private time to think. These kinds of diverse workspaces benefit introverts as well as extroverts, the systems design researcher Matt Davis told me, because they offer more spaces to retreat to than traditional open-plan offices.

I suspect that Wozniak himself would approve of these developments. Before he created the Apple PC, Woz designed calculators at Hewlett-Packard, a job he loved in part because HP made it so easy to chat with others. Every day at 10:00 a.m. and 2:00 p.m. management wheeled in donuts and coffee, and people would socialize and swap ideas. What set these interactions apart was how low-key and relaxed they were. In iWoz, he recalls HP as a meritocracy where it didn't matter what you looked like, where there was no premium on playing social games, and where no one pushed him from his beloved engineering work into management. That was what collaboration meant for Woz: the ability to share a donut and a brainwave with his laid-back, nonjudgmental, poorly dressed colleagues—who minded not a whit when he disappeared into his cubicle to get the real work done.

Part

TWO

YOUR BIOLOGY, YOUR SELF?

4

IS TEMPERAMENT DESTINY?

Nature, Nurture, and the Orchid Hypothesis

Some people are more certain of everything than I am of anything.

—ROBERT RUBIN, In an Uncertain World

ALMOST TEN YEARS AGO

It's 2:00 a.m., I can't sleep, and I want to die.

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 ass 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-assurance 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 communiqués 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, documenting 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 lustily 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 lusty 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, 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?

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 Nazi 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 classic 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.