We might call this the "rubber band theory" of personality. We are like rubber bands at rest. We are elastic and can stretch ourselves, but only so much.

To understand why this might be so for high-reactives, it helps to look at what happens in the brain when we greet a stranger at a c.o.c.ktail party. Remember that the amygdala, and the limbic system of which it's a key part, is an ancient part of the brain-so old that primitive mammals have their own versions of this system. But as mammals became more complex, an area of the brain called the neocortex developed around the limbic system. The neocortex, and particularly the frontal cortex in humans, performs an astonishing array of functions, from deciding which brand of toothpaste to buy, to planning a meeting, to pondering the nature of reality. One of these functions is to soothe unwarranted fears.

If you were a high-reactive baby, then your amygdala may, for the rest of your life, go a bit wild every time you introduce yourself to a stranger at a c.o.c.ktail party. But if you feel relatively skilled in company, that's partly because your frontal cortex is there to tell you to calm down, extend a handshake, and smile. In fact, a recent fMRI study shows that when people use self-talk to rea.s.sess upsetting situations, activity in their prefrontal cortex increases in an amount correlated with a decrease of activity in their amygdala.

But the frontal cortex isn't all-powerful; it doesn't switch the amygdala off altogether. In one study, scientists conditioned a rat to a.s.sociate a certain sound with an electrical shock. Then they played that sound over and over again without administering the shock, until the rats lost their fear.

But it turned out that this "unlearning" was not as complete as the scientists first thought. When they severed the neural connections between the rats' cortex and amygdala, the rats became afraid of the sound again. This was because the fear conditioning had been suppressed by the activity of the cortex, but was still present in the amygdala. In humans with unwarranted fears, like batophobia, or fear of heights, the same thing happens. Repeated trips to the top of the Empire State Building seem to extinguish the fear, but it may come roaring back during times of stress-when the cortex has other things to do than soothe an excitable amygdala.

This helps explain why many high-reactive kids retain some of the fearful aspects of their temperament all the way into adulthood, no matter how much social experience they acquire or free will they exercise. My colleague Sally is a good example of this phenomenon. Sally is a thoughtful and talented book editor, a self-described shy introvert, and one of the most charming and articulate people I know. If you invite her to a party, and later ask your other guests whom they most enjoyed meeting, chances are they'll mention Sally. She's so sparkly, they'll tell you. So witty! So adorable!

Sally is conscious of how well she comes across-you can't be as appealing as she is without being aware of it. But that doesn't mean her amygdala knows it. When she arrives at a party, Sally often wishes she could hide behind the nearest couch-until her prefrontal cortex takes over and she remembers what a good conversationalist she is. Even so, her amygdala, with its lifetime of stored a.s.sociations between strangers and anxiety, sometimes prevails. Sally admits that sometimes she drives an hour to a party and then leaves five minutes after arriving.

When I think of my own experiences in light of Schwartz's findings, I realize it's not true that I'm no longer shy; I've just learned to talk myself down from the ledge (thank you, prefrontal cortex!). By now I do it so automatically that I'm hardly aware it's happening. When I talk with a stranger or a group of people, my smile is bright and my manner direct, but there's a split second that feels like I'm stepping onto a high wire. By now I've had so many thousands of social experiences that I've learned that the high wire is a figment of my imagination, or that I won't die if I fall. I rea.s.sure myself so instantaneously that I'm barely aware I'm doing it. But the rea.s.surance process is still happening-and occasionally it doesn't work. The word that Kagan first used to describe high-reactive people was inhibited, and that's exactly how I still feel at some dinner parties.

This ability to stretch ourselves-within limits-applies to extroverts, too. One of my clients, Alison, is a business consultant, mother, and wife with the kind of extroverted personality-friendly, forthright, perpetually on the go-that makes people describe her as a "force of nature." She has a happy marriage, two daughters she adores, and her own consulting firm that she built from scratch. She's rightly proud of what she's accomplished in life.

But she hasn't always felt so satisfied. The year she graduated from high school, she took a good look at herself and didn't like what she saw. Alison is extremely bright, but you couldn't see that from her high school transcript. She'd had her heart set on attending an Ivy League school, and had thrown that chance away.

And she knew why. She'd spent high school socializing-Alison was involved in practically every extracurricular activity her school had to offer-and that didn't leave much time for academics. Partly she blamed her parents, who were so proud of their daughter's social gifts that they hadn't insisted she study more. But mostly she blamed herself.

As an adult, Alison is determined not to make similar mistakes. She knows how easy it would be to lose herself in a whirl of PTA meetings and business networking. So Alison's solution is to look to her family for adaptive strategies. She happens to be the only child of two introverted parents, to be married to an introvert, and to have a younger daughter who is a strong introvert herself.

Alison has found ways to tap into the wavelength of the quiet types around her. When she visits her parents, she finds herself meditating and writing in her journal, just the way her mother does. At home she relishes peaceful evenings with her homebody husband. And her younger daughter, who enjoys intimate backyard talks with her mother, has Alison spending her afternoons engaged in thoughtful conversation.

Alison has even created a network of quiet, reflective friends. Although her best friend in the world, Amy, is a highly charged extrovert just like her, most of her other friends are introverts. "I so appreciate people who listen well," says Alison. "They are the friends I go have coffee with. They give me the most spot-on observations. Sometimes I haven't even realized I was doing something counterproductive, and my introverted friends will say, 'Here's what you're doing, and here are fifteen examples of when you did that same thing,' whereas my friend Amy wouldn't even notice. But my introverted friends are sitting back and observing, and we can really connect over that."

Alison remains her boisterous self, but she has also discovered how to be, and to benefit from, quiet.

Even though we can reach for the outer limits of our temperaments, it can often be better to situate ourselves squarely inside our comfort zones.

Consider the story of my client Esther, a tax lawyer at a large corporate law firm. A tiny brunette with a springy step and blue eyes as bright as headlamps, Esther was not shy and never had been. But she was decidedly introverted. Her favorite part of the day was the quiet ten minutes when she walked to the bus along the tree-lined streets of her neighborhood. Her second favorite part was when she got to close the door to her office and dig into her work.

Esther had chosen her career well. A mathematician's daughter, she loved to think about intimidatingly complex tax problems, and could discuss them with ease. (In chapter 7, I examine why introverts are so good at complex, focused problem-solving.) She was the youngest member of a close-knit working group operating inside a much larger law firm. This group comprised five other tax lawyers, all of whom supported one another's careers. Esther's work consisted of thinking deeply about questions that fascinated her and working closely with trusted colleagues.

But it happened that Esther's small group of tax lawyers periodically had to give presentations to the rest of the law firm. These talks were a source of misery for Esther, not because she was afraid of public speaking, but because she wasn't comfortable speaking extemporaneously. Esther's colleagues, in contrast-all of whom happened to be extroverts-were spontaneous talkers who decided what they'd say on their way to the presentation and were somehow able to convey their thoughts intelligibly and engagingly by the time they arrived.

Esther was fine if given a chance to prepare, but sometimes her colleagues failed to mention that they'd be delivering a talk until she arrived at work that morning. She a.s.sumed that their ability to speak improvisationally was a function of their superior understanding of tax law and that, as she gained more experience, she too would be able to "wing it." But as Esther became more senior and more knowledgeable, she still couldn't do it.

To solve Esther's problem, let's focus on another difference between introverts and extroverts: their preference for stimulation.

For several decades, beginning in the late 1960s, an influential research psychologist named Hans Eysenck hypothesized that human beings seek "just right" levels of stimulation-not too much and not too little. Stimulation is the amount of input we have coming in from the outside world. It can take any number of forms, from noise to social life to flashing lights. Eysenck believed that extroverts prefer more stimulation than introverts do, and that this explained many of their differences: introverts enjoy shutting the doors to their offices and plunging into their work, because for them this sort of quiet intellectual activity is optimally stimulating, while extroverts function best when engaged in higher-wattage activities like organizing team-building workshops or chairing meetings.

Eysenck also thought that the basis of these differences might be found in a brain structure called the ascending reticular activating system (ARAS). The ARAS is a part of the brain stem that has connections leading up to the cerebral cortex and other parts of the brain. The brain has excitatory mechanisms that cause us to feel awake, alert, and energetic-"aroused," in the parlance of psychologists. It also has calming mechanisms that do the opposite. Eysenck speculated that the ARAS regulates the balance between over- and under-arousal by controlling the amount of sensory stimulation that flows into the brain; sometimes the channels are wide open, so a lot of stimulation can get in, and sometimes they're constricted, so the brain is less stimulated. Eysenck thought that the ARAS functioned differently in introverts and extroverts: introverts have wide-open information channels, causing them to be flooded with stimulation and over-aroused, while extroverts have tighter channels, making them p.r.o.ne to under-arousal. Over-arousal doesn't produce anxiety so much as the sense that you can't think straight-that you've had enough and would like to go home now. Under-arousal is something like cabin fever. Not enough is happening: you feel itchy, restless, and sluggish, like you need to get out of the house already.

Today we know that the reality is far more complex. For one thing, the ARAS doesn't turn stimulation on and off like a fire truck's hose, flooding the entire brain at once; different parts of the brain are aroused more than others at different times. Also, high arousal levels in the brain don't always correlate with how aroused we feel. And there are many different kinds of arousal: arousal by loud music is not the same as arousal by mortar fire, which is not the same as arousal by presiding at a meeting; you might be more sensitive to one form of stimulation than to another. It's also too simple to say that we always seek moderate levels of arousal: excited fans at a soccer game crave hyperstimulation, while people who visit spas for relaxation treatments seek low levels.

Still, more than a thousand studies conducted by scientists worldwide have tested Eysenck's theory that cortical arousal levels are an important clue to the nature of introversion and extroversion, and it appears to be what the personality psychologist David Funder calls "half-right"-in very important ways. Whatever the underlying cause, there's a host of evidence that introverts are more sensitive than extroverts to various kinds of stimulation, from coffee to a loud bang to the dull roar of a networking event-and that introverts and extroverts often need very different levels of stimulation to function at their best.

In one well-known experiment, dating all the way back to 1967 and still a favorite in-cla.s.s demonstration in psychology courses, Eysenck placed lemon juice on the tongues of adult introverts and extroverts to find out who salivated more. Sure enough, the introverts, being more aroused by sensory stimuli, were the ones with the watery mouths.

In another famous study, introverts and extroverts were asked to play a challenging word game in which they had to learn, through trial and error, the governing principle of the game. While playing, they wore headphones that emitted random bursts of noise. They were asked to adjust the volume of their headsets up or down to the level that was "just right." On average, the extroverts chose a noise level of 72 decibels, while the introverts selected only 55 decibels. When working at the volume that they had selected-loud for the extroverts, quiet for the introverts-the two types were about equally aroused (as measured by their heart rates and other indicators). They also played equally well.

When the introverts were asked to work at the noise level preferred by the extroverts, and vice versa, everything changed. Not only were the introverts over-aroused by the loud noise, but they also underperformed-taking an average of 9.1 trials rather than 5.8 to learn the game. The opposite was true for the extroverts-they were under-aroused (and possibly bored) by the quieter conditions, and took an average of 7.3 trials, compared with the 5.4 they'd averaged under noisier conditions.

When combined with Kagan's findings on high reactivity, this line of studies offers a very empowering lens through which to view your personality. Once you understand introversion and extroversion as preferences for certain levels of stimulation, you can begin consciously trying to situate yourself in environments favorable to your own personality-neither overstimulating nor understimulating, neither boring nor anxiety-making. You can organize your life in terms of what personality psychologists call "optimal levels of arousal" and what I call "sweet spots," and by doing so feel more energetic and alive than before.

Your sweet spot is the place where you're optimally stimulated. You probably seek it out already without being aware that you're doing so. Imagine that you're lying contentedly in a hammock reading a great novel. This is a sweet spot. But after half an hour you realize that you've read the same sentence five times; now you're understimulated. So you call a friend and go out for brunch-in other words, you ratchet up your stimulation level-and as you laugh and gossip over blueberry pancakes, you're back, thank goodness, inside your sweet spot. But this agreeable state lasts only until your friend-an extrovert who needs much more stimulation than you do-persuades you to accompany her to a block party, where you're now confronted by loud music and a sea of strangers.

Your friend's neighbors seem affable enough, but you feel pressured to make small talk above the din of music. Now-bang, just like that-you've fallen out of your sweet spot, except this time you're overstimulated. And you'll probably feel that way until you pair off with someone on the periphery of the party for an in-depth conversation, or bow out altogether and return to your novel.

Imagine how much better you'll be at this sweet-spot game once you're aware of playing it. You can set up your work, your hobbies, and your social life so that you spend as much time inside your sweet spot as possible. People who are aware of their sweet spots have the power to leave jobs that exhaust them and start new and satisfying businesses. They can hunt for homes based on the temperaments of their family members-with cozy window seats and other nooks and crannies for the introverts, and large, open living-dining s.p.a.ces for the extroverts.

Understanding your sweet spot can increase your satisfaction in every arena of your life, but it goes even further than that. Evidence suggests that sweet spots can have life-or-death consequences. According to a recent study of military personnel conducted through the Walter Reed Army Inst.i.tute of Research, introverts function better than extroverts when sleep deprived, which is a cortically de-arousing condition (because losing sleep makes us less alert, active, and energetic). Drowsy extroverts behind the wheel should be especially careful-at least until they increase their arousal levels by chugging coffee or cranking up the radio. Conversely, introverts driving in loud, overly arousing traffic noise should work to stay focused, since the noise may impair their thinking.

Now that we know about optimal levels of stimulation, Esther's problem-winging it at the podium-also makes sense. Overarousal interferes with attention and short-term memory-key components of the ability to speak on the fly. And since public speaking is an inherently stimulating activity-even for those, like Esther, who suffer no stage fright-introverts can find their attention impaired just when they need it most. Esther could live to be a one-hundred-year-old lawyer, in other words, the most knowledgeable pract.i.tioner in her field, and she might never be comfortable speaking extemporaneously. She might find herself perpetually unable, at speech time, to draw on the ma.s.sive body of data sitting inside her long-term memory.

But once Esther understands herself, she can insist to her colleagues that they give her advance notice of any speaking events. She can practice her speeches and find herself well inside her sweet spot when finally she reaches the podium. She can prepare the same way for client meetings, networking events, even casual meetings with her colleagues-any situation of heightened intensity in which her short-term memory and the ability to think on her feet might be a little more compromised than usual.

Esther managed to solve her problem from the comfort of her sweet spot. Yet sometimes stretching beyond it is our only choice. Some years ago I decided that I wanted to conquer my fear of public speaking. After much hemming and hawing, I signed up for a workshop at the Public SpeakingSocial Anxiety Center of New York. I had my doubts; I felt like a garden-variety shy person, and I didn't like the pathological sound of the term "social anxiety." But the cla.s.s was based on desensitization training, an approach that made sense to me. Often used as a way to conquer phobias, desensitization involves exposing yourself (and your amygdala) to the thing you're afraid of over and over again, in manageable doses. This is very different from the well-meaning but unhelpful advice that you should just jump in at the deep end and try to swim-an approach that might work, but more likely will produce panic, further encoding in your brain a cycle of dread, fear, and shame.

I found myself in good company. There were about fifteen people in the cla.s.s, which was led by Charles di Cagno, a wiry, compact man with warm brown eyes and a sophisticated sense of humor. Charles is himself a veteran of exposure therapy. Public speaking anxiety doesn't keep him up at night anymore, he says, but fear is a wily enemy and he's always working to get the better of it.

The workshop had been in session for a few weeks before I joined, but Charles a.s.sured me that newcomers were welcome. The group was more diverse than I expected. There was a fashion designer with long, curly hair, bright lipstick, and pointy snakeskin boots; a secretary with thick gla.s.ses and a clipped, matter-of-fact manner, who talked a lot about her Mensa membership; a couple of investment bankers, tall and athletic; an actor with black hair and vivid blue eyes who bounded cheerfully across the room in his Puma sneakers but claimed to be terrified the entire time; a Chinese software designer with a sweet smile and a nervous laugh. A regular cross-section of New Yorkers, really. It might have been a cla.s.s in digital photography or Italian cooking.

Except that it wasn't. Charles explained that each of us would speak in front of the group, but at an anxiety level we could handle.

A martial arts instructor named Lateesha was first up that evening. Lateesha's a.s.signment was to read aloud to the cla.s.s from a Robert Frost poem. With her dreadlocks and wide smile, Lateesha looked as if she wasn't afraid of anything. But as she got ready to speak, her book propped open at the podium, Charles asked how anxious she was, on a scale of 1 to 10.

"At least seven," said Lateesha.

"Take it slow," he said. "There are only a few people out there who can completely overcome their fears, and they all live in Tibet."