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 spaces 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 Institute 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 practitioner in her field, and she might never be comfortable speaking extemporaneously. She might find herself perpetually unable, at speech time, to draw on the massive 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 Speaking–Social 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 class 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 class, 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 assured 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 glasses 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 class 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 assignment was to read aloud to the class 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."

Lateesha read the poem clearly and quietly, with only the slightest tremor in her voice. When she was finished, Charles beamed proudly.

"Stand up please, Lisa," he said, addressing an attractive young marketing director with shiny black hair and a gleaming engagement ring. "It's your turn to offer feedback. Did Lateesha look nervous?"

"No," said Lisa.

"I was really scared, though," Lateesha said.

"Don't worry, no one could tell," Lisa assured her.

The others nodded their heads vigorously. Couldn't tell at all, they echoed. Lateesha sat down, looking pleased.

Next it was my turn. I stood at a makeshift podium—really a music stand—and faced the group. The only sound in the room was the ticking of the ceiling fan and the blare of traffic outside. Charles asked me to introduce myself. I took a deep breath.

"HELLOOO!!!!" I shouted, hoping to sound dynamic.

Charles looked alarmed. "Just be yourself," he said.

My first exercise was simple. All I had to do was answer a few questions that people called out: Where do you live? What do you do for a living? What did you do this weekend?

I answered the questions in my normal, soft-spoken way. The group listened carefully.

"Does anyone have any more questions for Susan?" asked Charles. The group shook their heads.

"Now, Dan," said Charles, nodding at a strapping red-haired fellow who looked like one of those CNBC journalists reporting directly from the New York Stock Exchange, "you're a banker and you have tough standards. Tell me, did Susan look nervous?"

"Not at all," said Dan.

The rest of the group nodded. Not nervous at all, they murmured—just as they had for Lateesha.

You seem so outgoing, they added.

You came across as really confident!

You're lucky because you never run out of things to say.

I sat down feeling pretty good about myself. But soon I saw that Lateesha and I weren't the only ones to get that kind of feedback. A few others did as well. "You looked so calm!" these speakers were told, to their visible relief. "No one would ever know if they didn't know! What are you doing in this class?"

At first I wondered why I prized these reassurances so highly. Then I realized that I was attending the workshop because I wanted to stretch myself to the outer limits of my temperament. I wanted to be the best and bravest speaker I could be. The reassurances were evidence that I was on my way toward achieving this goal. I suspected that the feedback I was getting was overly charitable, but I didn't care. What mattered was that I'd addressed an audience that had received me well, and I felt good about the experience. I had begun to desensitize myself to the horrors of public speaking.

Since then, I've done plenty of speaking, to groups of ten and crowds of hundreds. I've come to embrace the power of the podium. For me this involves taking specific steps, including treating every speech as a creative project, so that when I get ready for the big day, I experience that delving-deep sensation I enjoy so much. I also speak on topics that matter to me deeply, and have found that I feel much more centered when I truly care about my subject.

This isn't always possible, of course. Sometimes speakers need to talk about subjects that don't interest them much, especially at work. I believe this is harder for introverts, who have trouble projecting artificial enthusiasm. But there's a hidden advantage to this inflexibility: it can motivate us to make tough but worthwhile career changes if we find ourselves compelled to speak too often about topics that leave us cold. There is no one more courageous than the person who speaks with the courage of his convictions.

6

"FRANKLIN WAS A POLITICIAN, BUT ELEANOR SPOKE OUT OF CONSCIENCE"

Why Cool Is Overrated

A shy man no doubt dreads the notice of strangers, but can hardly be said to be afraid of them. He may be as bold as a hero in battle, and yet have no self-confidence about trifles in the presence of strangers.

—CHARLES DARWIN

Easter Sunday, 1939. The Lincoln Memorial. Marian Anderson, one of the most extraordinary singers of her generation, takes the stage, the statue of the sixteenth president rising up behind her. A regal woman with toffee-colored skin, she gazes at her audience of 75,000: men in brimmed hats, ladies in their Sunday best, a great sea of black and white faces. "My country 'tis of thee," she begins, her voice soaring, each word pure and distinct. "Sweet land of liberty." The crowd is rapt and tearful. They never thought this day would come to pass.

And it wouldn't have, without Eleanor Roosevelt. Earlier that year, Anderson had planned to sing at Constitution Hall in Washington, D.C., but the Daughters of the American Revolution, who owned the hall, rejected her because of her race. Eleanor Roosevelt, whose family had fought in the Revolution, resigned from the DAR, helped arrange for Anderson to sing at the Lincoln Memorial—and ignited a national firestorm. Roosevelt was not the only one to protest, but she brought political clout to the issue, risking her own reputation in the process.

For Roosevelt, who seemed constitutionally unable to look away from other people's troubles, such acts of social conscience were nothing unusual. But others appreciated how remarkable they were. "This was something unique," recalled the African-American civil rights leader James Farmer of Roosevelt's brave stand. "Franklin was a politician. He weighed the political consequences of every step that he took. He was a good politician, too. But Eleanor spoke out of conscience, and acted as a conscientious person. That was different."

It was a role she played throughout their life together: Franklin's adviser, Franklin's conscience. He may have chosen her for just this reason; in other ways they were such an unlikely pair.

They met when he was twenty. Franklin was her distant cousin, a sheltered Harvard senior from an upper-crust family. Eleanor was only nineteen, also from a moneyed clan, but she had chosen to immerse herself in the sufferings of the poor, despite her family's disapproval. As a volunteer at a settlement house on Manhattan's impoverished Lower East Side, she had met children who were forced to sew artificial flowers in windowless factories to the point of exhaustion. She took Franklin with her one day. He couldn't believe that human beings lived in such miserable conditions—or that a young woman of his own class had been the one to open his eyes to this side of America. He promptly fell in love with her.

But Eleanor wasn't the light, witty type he'd been expected to marry. Just the opposite: she was slow to laugh, bored by small talk, serious-minded, shy. Her mother, a fine-boned, vivacious aristocrat, had nicknamed her "Granny" because of her demeanor. Her father, the charming and popular younger brother of Theodore Roosevelt, doted on her when he saw her, but he was drunk most of the time, and died when Eleanor was nine. By the time Eleanor met Franklin, she couldn't believe that someone like him would be interested in her. Franklin was everything that she was not: bold and buoyant, with a wide, irrepressible grin, as easy with people as she was cautious. "He was young and gay and good looking," Eleanor recalled, "and I was shy and awkward and thrilled when he asked me to dance."

At the same time, many told Eleanor that Franklin wasn't good enough for her. Some saw him as a lightweight, a mediocre scholar, a frivolous man-about-town. And however poor Eleanor's own self-image, she did not lack for admirers who appreciated her gravitas. Some of her suitors wrote grudging letters of congratulations to Franklin when he won her hand. "I have more respect and admiration for Eleanor than any girl I have ever met," one letter-writer said. "You are mighty lucky. Your future wife is such as it is the privilege of few men to have," said another.

But public opinion was beside the point for Franklin and Eleanor. Each had strengths that the other craved—her empathy, his bravado. "E is an Angel," Franklin wrote in his journal. When she accepted his marriage proposal in 1903, he proclaimed himself the happiest man alive. She responded with a flood of love letters. They were married in 1905 and went on to have six children.

Despite the excitement of their courtship, their differences caused trouble from the start. Eleanor craved intimacy and weighty conversations; he loved parties, flirting, and gossip. The man who would declare that he had nothing to fear but fear itself could not understand his wife's struggles with shyness. When Franklin was appointed assistant secretary of the navy in 1913, the pace of his social life grew ever more frenzied and the settings more gilded—elite private clubs, his Harvard friends' mansions. He caroused later and later into the night. Eleanor went home earlier and earlier.

In the meantime, Eleanor found herself with a full calendar of social duties. She was expected to pay visits to the wives of other Washington luminaries, leaving calling cards at their doors and holding open houses in her own home. She didn't relish this role, so she hired a social secretary named Lucy Mercer to help her. Which seemed a good idea—until the summer of 1917, when Eleanor took the children to Maine for the summer, leaving Franklin behind in Washington with Mercer. The two began a lifelong affair. Lucy was just the kind of lively beauty Franklin had been expected to marry in the first place.

Eleanor found out about Franklin's betrayal when she stumbled on a packet of love letters in his suitcase. She was devastated, but stayed in the marriage. And although they never rekindled the romantic side of their relationship, she and Franklin replaced it with something formidable: a union of his confidence with her conscience.

Fast-forward to our own time, where we'll meet another woman of similar temperament, acting out of her own sense of conscience. Dr. Elaine Aron is a research psychologist who, since her first scientific publication in 1997, has singlehandedly reframed what Jerome Kagan and others call high reactivity (and sometimes "negativity" or "inhibition"). She calls it "sensitivity," and along with her new name for the trait, she's transformed and deepened our understanding of it.

When I hear that Aron will be the keynote speaker at an annual weekend gathering of "highly sensitive people" at Walker Creek Ranch in Marin County, California, I quickly buy plane tickets. Jacquelyn Strickland, a psychotherapist and the founder and host of the event, explains that she created these weekends so that sensitive people could benefit from being in one another's presence. She sends me an agenda explaining that we'll be sleeping in rooms designated for "napping, journaling, puttering, meditating, organizing, writing, and reflecting."

"Please do socialize very quietly in your room (with consent of your roommate), or preferably in the group areas on walks and at mealtimes," says the agenda. The conference is geared to people who enjoy meaningful discussions and sometimes "move a conversation to a deeper level, only to find out we are the only ones there." There will be plenty of time for serious talk this weekend, we're assured. But we'll also be free to come and go as we please. Strickland knows that most of us will have weathered a lifetime of mandatory group activities, and she wants to show us a different model, if only for a few days.

Walker Creek Ranch sits on 1,741 acres of unspoiled Northern California wilderness. It offers hiking trails and wildlife and vast crystalline skies, but at its center is a cozy, barnlike conference center where about thirty of us gather on a Thursday afternoon in the middle of June. The Buckeye Lodge is outfitted with grey industrial carpets, large whiteboards, and picture windows overlooking sunny redwood forests. Alongside the usual piles of registration forms and name badges, there's a flip chart where we're asked to write our name and Myers-Briggs personality type. I scan the list. Everyone's an introvert except for Strickland, who is warm, welcoming, and expressive. (According to Aron's research, the majority, though not all, of sensitive people are introverts.)

The tables and chairs in the room are organized in a big square so that we can all sit and face one another. Strickland invites us—participation optional—to share what brought us here. A software engineer named Tom kicks off, describing with great passion his relief at learning that there was "a physiological basis for the trait of sensitivity. Here's the research! This is how I am! I don't have to try to meet anyone's expectations anymore. I don't need to feel apologetic or defensive in any way." With his long, narrow face, brown hair, and matching beard, Tom reminds me of Abraham Lincoln. He introduces his wife, who talks about how compatible she and Tom are, and how together they stumbled across Aron's work.

When it's my turn, I talk about how I've never been in a group environment in which I didn't feel obliged to present an unnaturally rah-rah version of myself. I say that I'm interested in the connection between introversion and sensitivity. Many people nod.

On Saturday morning, Dr. Aron appears in the Buckeye Lodge. She waits playfully behind an easel containing a flip chart while Strickland introduces her to the audience. Then she emerges smiling—ta-da!—from behind the easel, sensibly clad in a blazer, turtleneck, and corduroy skirt. She has short, feathery brown hair and warm, crinkly blue eyes that look as if they don't miss a thing. You can see immediately the dignified scholar Aron is today, as well as the awkward schoolgirl she must once have been. You can see, too, her respect for her audience.

Getting right down to business, she informs us that she has five different subtopics she can discuss, and asks us to raise our hands to vote for our first, second, and third choice of subjects. Then she performs, rapid-fire, an elaborate mathematical calculation from which she determines the three subtopics for which we've collectively voted. The crowd settles down amiably. It doesn't really matter which subtopics we've chosen; we know that Aron is here to talk about sensitivity, and that she's taking our preferences into consideration.

Some psychologists make their mark by doing unusual research experiments. Aron's contribution is to think differently, radically differently, about studies that others have done. When she was a girl, Aron was often told that she was "too sensitive for her own good." She had two hardy elder siblings and was the only child in her family who liked to daydream, and play inside, and whose feelings were easily hurt. As she grew older and ventured outside her family's orbit, she continued to notice things about herself that seemed different from the norm. She could drive alone for hours and never turn on the radio. She had strong, sometimes disturbing dreams at night. She was "strangely intense," and often beset by powerful emotions, both positive and negative. She had trouble finding the sacred in the everyday; it seemed to be there only when she withdrew from the world.

Aron grew up, became a psychologist, and married a robust man who loved these qualities. To her husband, Art, Aron was creative, intuitive, and a deep thinker. She appreciated these things in herself, too, but saw them as "acceptable surface manifestations of a terrible, hidden flaw I had been aware of all my life." She thought it was a miracle that Art loved her in spite of this flaw.

But when one of her fellow psychologists casually described Aron as "highly sensitive," a lightbulb went on in her head. It was as if these two words described her mysterious failing, except that the psychologist hadn't been referring to a flaw at all. It had been a neutral description.

Aron pondered this new insight, and then set out to research this trait called "sensitivity." She came up mostly dry, so she pored over the vast literature on introversion, which seemed to be intimately related: Kagan's work on high-reactive children, and the long line of experiments on the tendency of introverts to be more sensitive to social and sensory stimulation. These studies gave her glimpses of what she was looking for, but Aron thought that there was a missing piece in the emerging portrait of introverted people.

"The problem for scientists is that we try to observe behavior, and these are things that you cannot observe," she explains. Scientists can easily report on the behavior of extroverts, who can often be found laughing, talking, or gesticulating. But "if a person is standing in the corner of a room, you can attribute about fifteen motivations to that person. But you don't really know what's going on inside."

Yet inner behavior was still behavior, thought Aron, even if it was difficult to catalog. So what is the inner behavior of people whose most visible feature is that when you take them to a party they aren't very pleased about it? She decided to find out.

First Aron interviewed thirty-nine people who described themselves as being either introverted or easily overwhelmed by stimulation. She asked them about the movies they liked, their first memories, relationships with parents, friendships, love lives, creative activities, philosophical and religious views. Based on these interviews, she created a voluminous questionnaire that she gave to several large groups of people. Then she boiled their responses down to a constellation of twenty-seven attributes. She named the people who embodied these attributes "highly sensitive."

Some of these twenty-seven attributes were familiar from Kagan and others' work. For example, highly sensitive people tend to be keen observers who look before they leap. They arrange their lives in ways that limit surprises. They're often sensitive to sights, sounds, smells, pain, coffee. They have difficulty when being observed (at work, say, or performing at a music recital) or judged for general worthiness (dating, job interviews).

But there were also new insights. The highly sensitive tend to be philosophical or spiritual in their orientation, rather than materialistic or hedonistic. They dislike small talk. They often describe themselves as creative or intuitive (just as Aron's husband had described her). They dream vividly, and can often recall their dreams the next day. They love music, nature, art, physical beauty. They feel exceptionally strong emotions—sometimes acute bouts of joy, but also sorrow, melancholy, and fear.

Highly sensitive people also process information about their environments—both physical and emotional—unusually deeply. They tend to notice subtleties that others miss—another person's shift in mood, say, or a lightbulb burning a touch too brightly.

Recently a group of scientists at Stony Brook University tested this finding by showing two pairs of photos (of a fence and some bales of hay) to eighteen people lying inside fMRI machines. In one pair the photos were noticeably different from each other, and in the other pair the difference was much more subtle. For each pair, the scientists asked whether the second photo was the same as the first. They found that sensitive people spent more time than others looking at the photos with the subtle differences. Their brains also showed more activity in regions that help to make associations between those images and other stored information. In other words, the sensitive people were processing the photos at a more elaborate level than their peers, reflecting more on those fenceposts and haystacks.

This study is very new, and its conclusions still need to be replicated and explored in other contexts. But it echoes Jerome Kagan's findings that high-reactive first graders spend more time than other children comparing choices when they play matching games or reading unfamiliar words. And it suggests, says Jadzia Jagiellowicz, the lead scientist at Stony Brook, that sensitive types think in an unusually complex fashion. It may also help explain why they're so bored by small talk. "If you're thinking in more complicated ways," she told me, "then talking about the weather or where you went for the holidays is not quite as interesting as talking about values or morality."

The other thing Aron found about sensitive people is that sometimes they're highly empathic. It's as if they have thinner boundaries separating them from other people's emotions and from the tragedies and cruelties of the world. They tend to have unusually strong consciences. They avoid violent movies and TV shows; they're acutely aware of the consequences of a lapse in their own behavior. In social settings they often focus on subjects like personal problems, which others consider "too heavy."

Aron realized that she was on to something big. Many of the characteristics of sensitive people that she'd identified—such as empathy and responsiveness to beauty—were believed by psychologists to be characteristic of other personality traits like "agreeableness" and "openness to experience." But Aron saw that they were also a fundamental part of sensitivity. Her findings implicitly challenged accepted tenets of personality psychology.

She started publishing her results in academic journals and books, and speaking publicly about her work. At first this was difficult. Audience members told her that her ideas were fascinating, but that her uncertain delivery was distracting. But Aron had a great desire to get her message out. She persevered, and learned to speak like the authority she was. By the time I saw her at Walker Creek Ranch, she was practiced, crisp, and sure. The only difference between her and your typical speaker was how conscientious she seemed about answering every last audience question. She lingered afterward with the group, even though, as an extreme introvert, she must have been itching to get home.

Aron's description of highly sensitive people sounds as if she's talking about Eleanor Roosevelt herself. Indeed, in the years since Aron first published her findings, scientists have found that when you put people whose genetic profiles have been tentatively associated with sensitivity and introversion (people with the gene variant of 5-HTTLPR that characterized the rhesus monkeys of chapter 3) inside an fMRI machine and show them pictures of scared faces, accident victims, mutilated bodies, and polluted scenery, the amygdala—the part of the brain that plays such an important role in processing emotions—becomes strongly activated. Aron and a team of scientists have also found that when sensitive people see faces of people experiencing strong feelings, they have more activation than others do in areas of the brain associated with empathy and with trying to control strong emotions.

It's as if, like Eleanor Roosevelt, they can't help but feel what others feel.

In 1921, FDR contracted polio. It was a terrible blow, and he considered retiring to the country to live out his life as an invalid gentleman. But Eleanor kept his contacts with the Democratic Party alive while he recovered, even agreeing to address a party fund-raiser. She was terrified of public speaking, and not much good at it—she had a high-pitched voice and laughed nervously at all the wrong times. But she trained for the event and made her way through the speech.

After that, Eleanor was still unsure of herself, but she began working to fix the social problems she saw all around her. She became a champion of women's issues and forged alliances with other serious-minded people. By 1928, when FDR was elected governor of New York, she was the director of the Bureau of Women's Activities for the Democratic Party and one of the most influential women in American politics. She and Franklin were now a fully functioning partnership of his savoir faire and her social conscience. "I knew about social conditions, perhaps more than he did," Eleanor recalled with characteristic modesty. "But he knew about government and how you could use government to improve things. And I think we began to get an understanding of teamwork."

FDR was elected president in 1933. It was the height of the Depression, and Eleanor traveled the country—in a single three-month period she covered 40,000 miles—listening to ordinary people tell their hard-luck stories. People opened up to her in ways they didn't for other powerful figures. She became for Franklin the voice of the dispossessed. When she returned home from her trips, she often told him what she'd seen and pressed him to act. She helped orchestrate government programs for half-starved miners in Appalachia. She urged FDR to include women and African-Americans in his programs to put people back to work. And she helped arrange for Marian Anderson to sing at the Lincoln Memorial. "She kept at him on issues which he might, in the rush of things, have wanted to overlook," the historian Geoff Ward has said. "She kept him to a high standard. Anyone who ever saw her lock eyes with him and say, 'Now Franklin, you should ...' never forgot it."

The shy young woman who'd been terrified of public speaking grew to love public life. Eleanor Roosevelt became the first First Lady to hold a press conference, address a national convention, write a newspaper column, and appear on talk radio. Later in her career she served as a U.S. delegate to the United Nations, where she used her unusual brand of political skills and hard-won toughness to help win passage of the Universal Declaration of Human Rights.

She never did outgrow her vulnerability; all her life she suffered dark "Griselda moods," as she called them (named for a princess in a medieval legend who withdrew into silence), and struggled to "develop skin as tough as rhinoceros hide." "I think people who are shy remain shy always, but they learn how to overcome it," she said. But it was perhaps this sensitivity that made it easy for her to relate to the disenfranchised, and conscientious enough to act on their behalf. FDR, elected at the start of the Depression, is remembered for his compassion. But it was Eleanor who made sure he knew how suffering Americans felt.

The connection between sensitivity and conscience has long been observed. Imagine the following experiment, performed by the developmental psychologist Grazyna Kochanska. A kind woman hands a toy to a toddler, explaining that the child should be very careful because it's one of the woman's favorites. The child solemnly nods assent and begins to play with the toy. Soon afterward, it breaks dramatically in two, having been rigged to do so.

The woman looks upset and cries, "Oh my!" Then she waits to see what the child does next.

Some children, it turns out, feel a lot more guilty about their (supposed) transgression than others. They look away, hug themselves, stammer out confessions, hide their faces. And it's the kids we might call the most sensitive, the most high-reactive, the ones who are likely to be introverts who feel the guiltiest. Being unusually sensitive to all experience, both positive and negative, they seem to feel both the sorrow of the woman whose toy is broken and the anxiety of having done something bad. (In case you're wondering, the woman in the experiments quickly returned to the room with the toy "fixed" and reassurances that the child had done nothing wrong.)

In our culture, guilt is a tainted word, but it's probably one of the building blocks of conscience. The anxiety these highly sensitive toddlers feel upon apparently breaking the toy gives them the motivation to avoid harming someone's plaything the next time. By age four, according to Kochanska, these same kids are less likely than their peers to cheat or break rules, even when they think they can't be caught. And by six or seven, they're more likely to be described by their parents as having high levels of moral traits such as empathy. They also have fewer behavioral problems in general.

"Functional, moderate guilt," writes Kochanska, "may promote future altruism, personal responsibility, adaptive behavior in school, and harmonious, competent, and prosocial relationships with parents, teachers, and friends." This is an especially important set of attributes at a time when a 2010 University of Michigan study shows that college students today are 40 percent less empathetic than they were thirty years ago, with much of the drop having occurred since 2000. (The study's authors speculate that the decline in empathy is related to the prevalence of social media, reality TV, and "hyper-competitiveness.")

Of course, having these traits doesn't mean that sensitive children are angels. They have selfish streaks like everyone else. Sometimes they act aloof and unfriendly. And when they're overwhelmed by negative emotions like shame or anxiety, says Aron, they can be positively oblivious of other people's needs.

But the same receptivity to experience that can make life difficult for the highly sensitive also builds their consciences. Aron tells of one sensitive teen who persuaded his mother to feed a homeless person he'd met in the park, and of another eight-year-old who cried not only when she felt embarrassed, but also when her peers were teased.

We know this type of person well from literature, probably because so many writers are sensitive introverts themselves. He "had gone through life with one skin fewer than most men," the novelist Eric Malpass writes of his quiet and cerebral protagonist, also an author, in the novel The Long Long Dances. "The troubles of others moved him more, as did also the teeming beauty of life: moved him, compelled him, to seize a pen and write about them. [He was moved by] walking in the hills, listening to a Schubert impromptu, watching nightly from his armchair the smashing of bone and flesh that made up so much of the nine o'clock news."

The description of such characters as thin-skinned is meant metaphorically, but it turns out that it's actually quite literal. Among the tests researchers use to measure personality traits are skin conductance tests, which record how much people sweat in response to noises, strong emotions, and other stimuli. High-reactive introverts sweat more; low-reactive extroverts sweat less. Their skin is literally "thicker," more impervious to stimuli, cooler to the touch. In fact, according to some of the scientists I spoke to, this is where our notion of being socially "cool" comes from; the lower-reactive you are, the cooler your skin, the cooler you are. (Incidentally, sociopaths lie at the extreme end of this coolness barometer, with extremely low levels of arousal, skin conductance, and anxiety. There is some evidence that sociopaths have damaged amygdalae.)

Lie detectors (polygraphs) are partially skin conductance tests. They operate on the theory that lying causes anxiety, which triggers the skin to perspire imperceptibly. When I was in college, I applied for a summer job as a secretary at a large jewelry company. I had to take a lie detector test as part of the application process. The test was administered in a small, dingily lit room with linoleum floors, by a thin, cigarette-puffing man with pocked yellow skin. The man asked me a series of warm-up questions: my name, address, and so on, to establish my baseline level of skin conductance. Then the questions grew more probing and the examiner's manner harsher. Had I been arrested? Had I ever shoplifted? Had I used cocaine? With this last question my interrogator peered at me intently. As it happens, I never had tried cocaine. But he seemed to think I had. The accusing look on his face was the equivalent of the old policeman's trick where they tell the suspect that they have the damning evidence and there's no point denying it.

I knew the man was mistaken, but I still felt myself blush. And sure enough, the test came back showing I'd lied on the cocaine question. My skin is so thin, apparently, that it sweats in response to imaginary crimes!

We tend to think of coolness as a pose that you strike with a pair of sunglasses, a nonchalant attitude, and drink in hand. But maybe we didn't choose these social accessories at random. Maybe we've adopted dark glasses, relaxed body language, and alcohol as signifiers precisely because they camouflage signs of a nervous system on overdrive. Sunglasses prevent others from seeing our eyes dilate with surprise or fear; we know from Kagan's work that a relaxed torso is a hallmark of low reactivity; and alcohol removes our inhibitions and lowers our arousal levels. When you go to a football game and someone offers you a beer, says the personality psychologist Brian Little, "they're really saying hi, have a glass of extroversion."