In fact, public speaking anxiety may be primal and quintessentially human, not limited to those of us born with a high-reactive nervous system. One theory, based on the writings of the sociobiologist E. O. Wilson, holds that when our ancestors lived on the savannah, being watched intently meant only one thing: a wild animal was stalking us. And when we think we're about to be eaten, do we stand tall and hold forth confidently? No. We run. In other words, hundreds of thousands of years of evolution urge us to get the h.e.l.l off the stage, where we can mistake the gaze of the spectators for the glint in a predator's eye. Yet the audience expects not only that we'll stay put, but that we'll act relaxed and a.s.sured. This conflict between biology and protocol is one reason that speechmaking can be so fraught. It's also why exhortations to imagine the audience in the nude don't help nervous speakers; naked lions are just as dangerous as elegantly dressed ones.

But even though all human beings may be p.r.o.ne to mistaking audience members for predators, each of us has a different threshold for triggering the fight-or-flight response. How threateningly must the eyes of the audience members narrow before you feel they're about to pounce? Does it happen before you've even stepped onstage, or does it take a few really good hecklers to trigger that adrenaline rush? You can see how a highly sensitive amygdala would make you more susceptible to frowns and bored sighs and people who check their BlackBerrys while you're in mid-sentence. And indeed, studies do show that introverts are significantly more likely than extroverts to fear public speaking.

Kagan tells me about the time he watched a fellow scientist give a wonderful talk at a conference. Afterward, the speaker asked if they could have lunch. Kagan agreed, and the scientist proceeded to tell him that he gives lectures every month and, despite his capable stage persona, is terrified each time. Reading Kagan's work had had a big impact on him, however.

"You changed my life," he told Kagan. "All this time I've been blaming my mother, but now I think I'm a high-reactive."

So am I introverted because I inherited my parents' high reactivity, copied their behaviors, or both? Remember that the heritability statistics derived from twin studies show that introversion-extroversion is only 40 to 50 percent heritable. This means that, in a group of people, on average half of the variability in introversion-extroversion is caused by genetic factors. To make things even more complex, there are probably many genes at work, and Kagan's framework of high reactivity is likely one of many physiological routes to introversion. Also, averages are tricky. A heritability rate of 50 percent doesn't necessarily mean that my introversion is 50 percent inherited from my parents, or that half of the difference in extroversion between my best friend and me is genetic. One hundred percent of my introversion might come from genes, or none at all-or more likely some unfathomable combination of genes and experience. To ask whether it's nature or nurture, says Kagan, is like asking whether a blizzard is caused by temperature or humidity. It's the intricate interaction between the two that makes us who we are.

So perhaps I've been asking the wrong question. Maybe the mystery of what percent of personality is nature and what percent nurture is less important than the question of how your inborn temperament interacts with the environment and with your own free will. To what degree is temperament destiny?

On the one hand, according to the theory of gene-environment interaction, people who inherit certain traits tend to seek out life experiences that reinforce those characteristics. The most low-reactive kids, for example, court danger from the time they're toddlers, so that by the time they grow up they don't bat an eye at grown-up-sized risks. They "climb a few fences, become desensitized, and climb up on the roof," the late psychologist David Lykken once explained in an Atlantic article. "They'll have all sorts of experiences that other kids won't. Chuck Yeager (the first pilot to break the sound barrier) could step down from the belly of the bomber into the rocketship and push the b.u.t.ton not because he was born with that difference between him and me, but because for the previous thirty years his temperament impelled him to work his way up from climbing trees through increasing degrees of danger and excitement."

Conversely, high-reactive children may be more likely to develop into artists and writers and scientists and thinkers because their aversion to novelty causes them to spend time inside the familiar-and intellectually fertile-environment of their own heads. "The university is filled with introverts," observes the psychologist Jerry Miller, director of the Center for the Child and the Family at the University of Michigan. "The stereotype of the university professor is accurate for so many people on campus. They like to read; for them there's nothing more exciting than ideas. And some of this has to do with how they spent their time when they were growing up. If you spend a lot of time charging around, then you have less time for reading and learning. There's only so much time in your life."

On the other hand, there is also a wide range of possible outcomes for each temperament. Low-reactive, extroverted children, if raised by attentive families in safe environments, can grow up to be energetic achievers with big personalities-the Richard Bransons and Oprahs of this world. But give those same children negligent caregivers or a bad neighborhood, say some psychologists, and they can turn into bullies, juvenile delinquents, or criminals. Lykken has controversially called psychopaths and heroes "twigs on the same genetic branch."

Consider the mechanism by which kids acquire their sense of right and wrong. Many psychologists believe that children develop a conscience when they do something inappropriate and are rebuked by their caregivers. Disapproval makes them feel anxious, and since anxiety is unpleasant, they learn to steer clear of antisocial behavior. This is known as internalizing their parents' standards of conduct, and its core is anxiety.

But what if some kids are less p.r.o.ne to anxiety than others, as is true of extremely low-reactive kids? Often the best way to teach these children values is to give them positive role models and to channel their fearlessness into productive activities. A low-reactive child on an ice-hockey team enjoys his peers' esteem when he charges at his opponents with a lowered shoulder, which is a "legal" move. But if he goes too far, raises his elbow, and gives another guy a concussion, he lands in the penalty box. Over time he learns to use his appet.i.te for risk and a.s.sertiveness wisely.

Now imagine this same child growing up in a dangerous neighborhood with few organized sports or other constructive channels for his boldness. You can see how he might fall into delinquency. It may be that some disadvantaged kids who get into trouble suffer not solely from poverty or neglect, say those who hold this view, but also from the tragedy of a bold and exuberant temperament deprived of healthy outlets.

The destinies of the most high-reactive kids are also influenced by the world around them-perhaps even more so than for the average child, according to a groundbreaking new theory dubbed "the orchid hypothesis" by David Dobbs in a wonderful article in The Atlantic. This theory holds that many children are like dandelions, able to thrive in just about any environment. But others, including the high-reactive types that Kagan studied, are more like orchids: they wilt easily, but under the right conditions can grow strong and magnificent.

According to Jay Belsky, a leading proponent of this view and a psychology professor and child care expert at the University of London, the reactivity of these kids' nervous systems makes them quickly overwhelmed by childhood adversity, but also able to benefit from a nurturing environment more than other children do. In other words, orchid children are more strongly affected by all experience, both positive and negative.

Scientists have known for a while that high-reactive temperaments come with risk factors. These kids are especially vulnerable to challenges like marital tension, a parent's death, or abuse. They're more likely than their peers to react to these events with depression, anxiety, and shyness. Indeed, about a quarter of Kagan's high-reactive kids suffer from some degree of the condition known as "social anxiety disorder," a chronic and disabling form of shyness.

What scientists haven't realized until recently is that these risk factors have an upside. In other words, the sensitivities and the strengths are a package deal. High-reactive kids who enjoy good parenting, child care, and a stable home environment tend to have fewer emotional problems and more social skills than their lower-reactive peers, studies show. Often they're exceedingly empathic, caring, and cooperative. They work well with others. They are kind, conscientious, and easily disturbed by cruelty, injustice, and irresponsibility. They're successful at the things that matter to them. They don't necessarily turn into cla.s.s presidents or stars of the school play, Belsky told me, though this can happen, too: "For some it's becoming the leader of their cla.s.s. For others it takes the form of doing well academically or being well-liked."

The upsides of the high-reactive temperament have been doc.u.mented in exciting research that scientists are only now beginning to pull together. One of the most interesting findings, also reported in Dobbs's Atlantic article, comes from the world of rhesus monkeys, a species that shares about 95 percent of its DNA with humans and has elaborate social structures that resemble our own.

In these monkeys as well as in humans, a gene known as the serotonin-transporter (SERT) gene, or 5-HTTLPR, helps to regulate the processing of serotonin, a neurotransmitter that affects mood. A particular variation, or allele, of this gene, sometimes referred to as the "short" allele, is thought to be a.s.sociated with high reactivity and introversion, as well as a heightened risk of depression in humans who have had difficult lives. When baby monkeys with a similar allele were subjected to stress-in one experiment they were taken from their mothers and raised as orphans-they processed serotonin less efficiently (a risk factor for depression and anxiety) than monkeys with the long allele who endured similar privations. But young monkeys with the same risky genetic profile who were raised by nurturing mothers did as well as or better than their long-allele brethren-even those raised in similarly secure environments-at key social tasks, like finding playmates, building alliances, and handling conflicts. They often became leaders of their troops. They also processed serotonin more efficiently.

Stephen Suomi, the scientist who conducted these studies, has speculated that these high-reactive monkeys owed their success to the enormous amounts of time they spent watching rather than partic.i.p.ating in the group, absorbing on a deep level the laws of social dynamics. (This is a hypothesis that might ring true to parents whose high-reactive children hover observantly on the edges of their peer group, sometimes for weeks or months, before edging successfully inside.)

Studies in humans have found that adolescent girls with the short allele of the SERT gene are 20 percent more likely to be depressed than long-allele girls when exposed to stressful family environments, but 25 percent less likely to be depressed when raised in stable homes. Similarly, short allele adults have been shown to have more anxiety in the evening than others when they've had stressful days, but less anxiety on calm days. High-reactive four-year-olds give more pro-social responses than other children when presented with moral dilemmas-but this difference remains at age five only if their mothers used gentle, not harsh, discipline. High-reactive children raised in supportive environments are even more resistant than other kids to the common cold and other respiratory illnesses, but get sick more easily if they're raised in stressful conditions. The short allele of the SERT gene is also a.s.sociated with higher performance on a wide range of cognitive tasks.

These findings are so dramatic that it's remarkable no one arrived at them until recently. Remarkable, but perhaps not surprising. Psychologists are trained to heal, so their research naturally focuses on problems and pathology. "It is almost as if, metaphorically speaking, sailors are so busy-and wisely-looking under the water line for extensions of icebergs that could sink their ship," writes Belsky, "that they fail to appreciate that by climbing on top of the iceberg it might prove possible to chart a clear pa.s.sage through the ice-laden sea."

The parents of high-reactive children are exceedingly lucky, Belsky told me. "The time and effort they invest will actually make a difference. Instead of seeing these kids as vulnerable to adversity, parents should see them as malleable-for worse, but also for better." He describes eloquently a high-reactive child's ideal parent: someone who "can read your cues and respect your individuality; is warm and firm in placing demands on you without being harsh or hostile; promotes curiosity, academic achievement, delayed gratification, and self-control; and is not harsh, neglectful, or inconsistent." This advice is terrific for all parents, of course, but it's crucial for raising a high-reactive child. (If you think your child might be high-reactive, you're probably already asking yourself what else you can do to cultivate your son or daughter. Chapter 11 has some answers.)

But even orchid children can withstand some adversity, Belsky says. Take divorce. In general, it will disrupt orchid kids more than others: "If the parents squabble a lot, and put their kid in the middle, then watch out-this is the kid who will succ.u.mb." But if the divorcing parents get along, if they provide their child with the other psychological nutrients he needs, then even an orchid child can do just fine.

Most people would appreciate the flexibility of this message, I think; few of us had problem-free childhoods.

But there's another kind of flexibility that we all hope applies to the question of who we are and what we become. We want the freedom to map our own destinies. We want to preserve the advantageous aspects of our temperaments and improve, or even discard, the ones we dislike-such as a horror of public speaking. In addition to our inborn temperaments, beyond the luck of the draw of our childhood experience, we want to believe that we-as adults-can shape our selves and make what we will of our lives.

Can we?

CHAPTER 5

BEYOND TEMPERAMENT

The Role of Free Will (and the Secret of Public Speaking for Introverts)

Enjoyment appears at the boundary between boredom and anxiety, when the challenges are just balanced with the person's capacity to act.

-MIHALY CSIKSZENTMIHALYI

Deep inside the bowels of the Athinoula A. Martinos Center for Biomedical Imaging at Ma.s.sachusetts General Hospital, the hallways are nondescript, dingy even. I'm standing outside the locked door of a windowless room with Dr. Carl Schwartz, the director of the Developmental Neuroimaging and Psychopathology Research Lab. Schwartz has bright, inquisitive eyes, graying brown hair, and a quietly enthusiastic manner. Despite our unprepossessing surroundings, he prepares with some fanfare to unlock the door.

The room houses a multimillion-dollar fMRI (functional magnetic resonance imaging) machine, which has made possible some of the greatest breakthroughs in modern neuroscience. An fMRI machine can measure which parts of the brain are active when you're thinking a particular thought or performing a specific task, allowing scientists to perform the once unimaginable task of mapping the functions of the human brain. A princ.i.p.al inventor of the fMRI technique, says Dr. Schwartz, was a brilliant but una.s.suming scientist named Kenneth Kwong, who works inside this very building. This whole place is full of quiet and modest people doing extraordinary things, Schwartz adds, waving his hand appreciatively at the empty hallway.

Before Schwartz opens the door, he asks me to take off my gold hoop earrings and set aside the metal tape recorder I've been using to record our conversation. The magnetic field of the fMRI machine is 100,000 times stronger than the earth's gravitational pull-so strong, Schwartz says, that it could rip the earrings right out of my ears if they were magnetic and send them flying across the room. I worry about the metal fasteners of my bra, but I'm too embarra.s.sed to ask. I point instead to my shoe buckle, which I figure has the same amount of metal as the bra strap. Schwartz says it's all right, and we enter the room.

We gaze reverently at the fMRI scanner, which looks like a gleaming rocketship lying on its side. Schwartz explains that he asks his subjects-who are in their late teens-to lie down with their heads in the scanner while they look at photographs of faces and the machine tracks how their brains respond. He's especially interested in activity in the amygdala-the same powerful organ inside the brain that Kagan found played such an important role in shaping some introverts' and extroverts' personalities.

Schwartz is Kagan's colleague and protege, and his work picks up just where Kagan's longitudinal studies of personality left off. The infants Kagan once categorized as high- and low-reactive have now grown up, and Schwartz is using the fMRI machine to peer inside their brains. Kagan followed his subjects from infancy into adolescence, but Schwartz wanted to see what happened to them after that. Would the footprint of temperament be detectable, all those years later, in the adult brains of Kagan's high- and low-reactive infants? Or would it have been erased by some combination of environment and conscious effort?

Interestingly, Kagan cautioned Schwartz against doing the study. In the compet.i.tive field of science research, you don't want to waste time conducting studies that may not yield significant findings. And Kagan worried that there were no results to be found-that the link between temperament and destiny would be severed by the time an infant reached adulthood.

"He was trying to take care of me," Schwartz tells me. "It was an interesting paradox. Because here Jerry was doing all these early observations of infants, and seeing that it wasn't just their social behavior that was different in the extremes-everything about these kids was different. Their eyes dilated more widely when they were solving problems, their vocal cords became more tense while uttering words, their heart rate patterns were unique: there were all these channels that suggested there was something different physiologically about these kids. And I think, in spite of this, because of his intellectual heritage, he had the feeling that environmental factors are so complex that it would be really hard to pick up that footprint of temperament later in life."

But Schwartz, who believes that he's a high-reactive himself and was drawing partly on his own experience, had a hunch that he'd find that footprint even farther along the longitudinal timeline than Kagan had.

He demonstrates his research by allowing me to act as if I were one of his subjects, albeit not inside the fMRI scanner. As I sit at a desk, a computer monitor flashes photos at me, one after another, each showing an unfamiliar face: disembodied black-and-white heads floating against a dark background. I think I can feel my pulse quicken as the photos start coming at me faster and faster. I also notice that Schwartz has slipped in some repeats and that I feel more relaxed as the faces start to look familiar. I describe my reactions to Schwartz, who nods. The slide show is designed, he says, to mimic an environment that corresponds to the sense that high-reactive people get when they walk into a crowded room of strangers and feel "Geez! Who are these people?"

I wonder if I'm imagining my reactions, or exaggerating them, but Schwartz tells me that he's gotten back the first set of data on a group of high-reactive children Kagan studied from four months of age-and sure enough, the amygdalae of those children, now grown up, had turned out to be more sensitive to the pictures of unfamiliar faces than did the amygdalae of those who'd been bold toddlers. Both groups reacted to the pictures, but the formerly shy kids reacted more. In other words, the footprint of a high- or low-reactive temperament never disappeared in adulthood. Some high-reactives grew into socially fluid teenagers who were not outwardly rattled by novelty, but they never shed their genetic inheritance.

Schwartz's research suggests something important: we can stretch our personalities, but only up to a point. Our inborn temperaments influence us, regardless of the lives we lead. A sizable part of who we are is ordained by our genes, by our brains, by our nervous systems. And yet the elasticity that Schwartz found in some of the high-reactive teens also suggests the converse: we have free will and can use it to shape our personalities.

These seem like contradictory principles, but they are not. Free will can take us far, suggests Dr. Schwartz's research, but it cannot carry us infinitely beyond our genetic limits. Bill Gates is never going to be Bill Clinton, no matter how he polishes his social skills, and Bill Clinton can never be Bill Gates, no matter how much time he spends alone with a computer.