Teenagers understand instinctively the physiology of cool. In Curtis Sittenfeld's novel Prep, which explores the adolescent social rituals of boarding-school life with uncanny precision, the protagonist, Lee, is invited unexpectedly to the dorm room of Aspeth, the coolest girl in school. The first thing she notices is how physically stimulating Aspeth's world is. "From outside the door, I could hear pounding music," she observes. "White Christmas lights, currently turned on, were taped high up along all the walls, and on the north wall they'd hung an enormous orange and green tapestry.... I felt overstimulated and vaguely irritated. The room I shared with [my roommate] seemed so quiet and plain, our lives seemed so quiet and plain. Had Aspeth been born cool, I wondered, or had someone taught her, like an older sister or a cousin?"

Jock cultures sense the low-reactive physiology of cool, too. For the early U.S. astronauts, having a low heart rate, which is associated with low reactivity, was a status symbol. Lieutenant Colonel John Glenn, who became the first American to orbit the Earth and would later run for president, was admired by his comrades for his supercool pulse rate during liftoff (only 110 beats per minute).

But physical lack of cool may be more socially valuable than we think. That deep blush when a hard-bitten tester puts his face an inch from yours and asks if you've ever used cocaine turns out to be a kind of social glue. In a recent experiment, a team of psychologists led by Corine Dijk asked sixty-odd participants to read accounts of people who'd done something morally wrong, like driving away from a car crash, or something embarrassing, like spilling coffee on someone. The participants were shown photographs of the wrongdoers, who had one of four different facial expressions: shame or embarrassment (head and eyes down); shame/embarrassment plus a blush; neutral; or neutral with a blush. Then they were asked to rate how sympathetic and trustworthy the transgressors were.

It turned out that the offenders who blushed were judged a lot more positively than those who didn't. This was because the blush signified concern for others. As Dacher Keltner, a psychologist at the University of California, Berkeley, who specializes in positive emotions, put it to the New York Times, "A blush comes online in two or three seconds and says, 'I care; I know I violated the social contract.' "

In fact, the very thing that many high-reactives hate most about blushing—its uncontrollability—is what makes it so socially useful. "Because it is impossible to control the blush intentionally," Dijk speculates, blushing is an authentic sign of embarrassment. And embarrassment, according to Keltner, is a moral emotion. It shows humility, modesty, and a desire to avoid aggression and make peace. It's not about isolating the person who feels ashamed (which is how it sometimes feels to easy blushers), but about bringing people together.

Keltner has tracked the roots of human embarrassment and found that after many primates fight, they try to make up. They do this partly by making gestures of embarrassment of the kind we see in humans—looking away, which acknowledges wrongdoing and the intention to stop; lowering the head, which shrinks one's size; and pressing the lips together, a sign of inhibition. These gestures in humans have been called "acts of devotion," writes Keltner. Indeed, Keltner, who is trained in reading people's faces, has studied photos of moral heroes like Gandhi and the Dalai Lama and found that they feature just such controlled smiles and averted eyes.

In his book, Born to Be Good, Keltner even says that if he had to choose his mate by asking a single question at a speed-dating event, the question he would choose is: "What was your last embarrassing experience?" Then he would watch very carefully for lip-presses, blushing, and averted eyes. "The elements of the embarrassment are fleeting statements the individual makes about his or her respect for the judgment of others," he writes. "Embarrassment reveals how much the individual cares about the rules that bind us to one another."

In other words, you want to make sure that your spouse cares what other people think. It's better to mind too much than to mind too little.

No matter how great the benefits of blushing, the phenomenon of high sensitivity raises an obvious question. How did the highly sensitive manage to survive the harsh sorting-out process of evolution? If the bold and aggressive generally prevail (as it sometimes seems), why were the sensitive not selected out of the human population thousands of years ago, like tree frogs colored orange? For you may, like the protagonist of The Long Long Dances, be moved more deeply than the next person by the opening chords of a Schubert impromptu, and you may flinch more than others at the smashing of bone and flesh, and you may have been the sort of child who squirmed horribly when you thought you'd broken someone's toy, but evolution doesn't reward such things.

Or does it?

Elaine Aron has an idea about this. She believes that high sensitivity was not itself selected for, but rather the careful, reflective style that tends to accompany it. "The type that is 'sensitive' or 'reactive' would reflect a strategy of observing carefully before acting," she writes, "thus avoiding dangers, failures, and wasted energy, which would require a nervous system specially designed to observe and detect subtle differences. It is a strategy of 'betting on a sure thing' or 'looking before you leap.' In contrast, the active strategy of the [other type] is to be first, without complete information and with the attendant risks—the strategy of 'taking a long shot' because the 'early bird catches the worm' and 'opportunity only knocks once.' "

In truth, many people Aron considers sensitive have some of the twenty-seven attributes associated with the trait, but not all of them. Maybe they're sensitive to light and noise, but not to coffee or pain; maybe they're not sensitive to anything sensory, but they're deep thinkers with a rich inner life. Maybe they're not even introverts—only 70 percent of sensitive people are, according to Aron, while the other 30 percent are extroverts (although this group tends to report craving more downtime and solitude than your typical extrovert). This, speculates Aron, is because sensitivity arose as a by-product of survival strategy, and you need only some, not all, of the traits to pull off the strategy effectively.

There's a great deal of evidence for Aron's point of view. Evolutionary biologists once believed that every animal species evolved to fit an ecological niche, that there was one ideal set of behaviors for that niche, and that species members whose behavior deviated from that ideal would die off. But it turns out that it's not only humans that divide into those who "watch and wait" and others who "just do it." More than a hundred species in the animal kingdom are organized in roughly this way.

From fruit flies to house cats to mountain goats, from sunfish to bushbaby primates to Eurasian tit birds, scientists have discovered that approximately 20 percent of the members of many species are "slow to warm up," while the other 80 percent are "fast" types who venture forth boldly without noticing much of what's going on around them. (Intriguingly, the percentage of infants in Kagan's lab who were born high-reactive was also, you'll recall, about twenty.)

If "fast" and "slow" animals had parties, writes the evolutionary biologist David Sloan Wilson, "some of the fasts would bore everyone with their loud conversation, while others would mutter into their beer that they don't get any respect. Slow animals are best described as shy, sensitive types. They don't assert themselves, but they are observant and notice things that are invisible to the bullies. They are the writers and artists at the party who have interesting conversations out of earshot of the bullies. They are the inventors who figure out new ways to behave, while the bullies steal their patents by copying their behavior."

Once in a while, a newspaper or TV program runs a story about animal personalities, casting shy behavior as unseemly and bold behavior as attractive and admirable. (That's our kind of fruit fly!) But Wilson, like Aron, believes that both types of animals exist because they have radically different survival strategies, each of which pays off differently and at different times. This is what's known as the trade-off theory of evolution, in which a particular trait is neither all good nor all bad, but a mix of pros and cons whose survival value varies according to circumstance.

"Shy" animals forage less often and widely for food, conserving energy, sticking to the sidelines, and surviving when predators come calling. Bolder animals sally forth, swallowed regularly by those farther up the food chain but surviving when food is scarce and they need to assume more risk. When Wilson dropped metal traps into a pond full of pumpkinseed fish, an event he says must have seemed to the fish as unsettling as a flying saucer landing on Earth, the bold fish couldn't help but investigate—and rushed headlong into Wilson's traps. The shy fish hovered judiciously at the edge of the pond, making it impossible for Wilson to catch them.

On the other hand, after Wilson succeeded in trapping both types of fish with an elaborate netting system and carrying them back to his lab, the bold fish acclimated quickly to their new environment and started eating a full five days earlier than did their shy brethren. "There is no single best ... [animal] personality," writes Wilson, "but rather a diversity of personalities maintained by natural selection."

Another example of the trade-off theory of evolution is a species known as Trinidadian guppies. These guppies develop personalities—with astonishing speed, in evolutionary terms—to suit the microclimates in which they live. Their natural predators are pike. But some guppy neighborhoods, upstream of a waterfall for example, are pike-free. If you're a guppy who grew up in such a charmed locale, then chances are you have a bold and carefree personality well suited to la dolce vita. In contrast, if your guppy family came from a "bad neighborhood" downstream from the waterfall, where pike cruise the waterways menacingly, then you probably have a much more circumspect style, just right for avoiding the bad guys.

The interesting thing is that these differences are heritable, not learned, so that the offspring of bold guppies who move into bad neighborhoods inherit their parents' boldness—even though this puts them at a severe disadvantage compared to their vigilant peers. It doesn't take long for their genes to mutate, though, and descendants who manage to survive tend to be careful types. The same thing happens to vigilant guppies when the pike suddenly disappear; it takes about twenty years for their descendants to evolve into fish who act as if they haven't a care in the world.

The trade-off theory seems to apply equally to humans. Scientists have found that nomads who inherited the form of a particular gene linked to extroversion (specifically, to novelty-seeking) are better nourished than those without this version of the gene. But in settled populations, people with this same gene form have poorer nutrition. The same traits that make a nomad fierce enough to hunt and to defend livestock against raiders may hinder more sedentary activities like farming, selling goods at the market, or focusing at school.

Or consider this trade-off: human extroverts have more sex partners than introverts do—a boon to any species wanting to reproduce itself—but they commit more adultery and divorce more frequently, which is not a good thing for the children of all those couplings. Extroverts exercise more, but introverts suffer fewer accidents and traumatic injuries. Extroverts enjoy wider networks of social support, but commit more crimes. As Jung speculated almost a century ago about the two types, "the one [extroversion] consists in a high rate of fertility, with low powers of defense and short duration of life for the single individual; the other [introversion] consists in equipping the individual with numerous means of self-preservation plus a low fertility rate."

The trade-off theory may even apply to entire species. Among evolutionary biologists, who tend to subscribe to the vision of lone individuals hell-bent on reproducing their own DNA, the idea that species include individuals whose traits promote group survival is hotly debated and, not long ago, could practically get you kicked out of the academy. But this view is slowly gaining acceptance. Some scientists even speculate that the evolutionary basis for traits like sensitivity is heightened compassion for the suffering of other members of one's species, especially one's family.

But you don't have to go that far. As Aron explains, it makes sense that animal groups depend on their sensitive members for survival. "Suppose a herd of antelope ... has a few members who are constantly stopping their grazing to use their keen senses to watch for predators," she writes. "Herds with such sensitive, watchful individuals would survive better, and so continue to breed, and so continue to have some sensitive individuals born in the group."

And why should it be any different for humans? We need our Eleanor Roosevelts as surely as grazing herds depend on their sensitive antelopes.

In addition to "shy" and "bold" animals, and to "fast" and "slow" ones, biologists sometimes speak of the "hawk" and "dove" members of a given species. Great tit birds, for example, some of whom are much more aggressive than others, often act like case studies in an international relations class. These birds feed on beech tree nuts, and in years when nuts are scarce, the hawkish female birds do better, just as you'd expect, because they're quick to challenge nut-eating competitors to a duel. But in seasons when there are plenty of beech nuts to go around, the female "doves"—who, incidentally, tend to make more attentive mothers—do better than the "hawks," because the hawks waste time and bodily health getting into fights for no good reason.

Male great tits, on the other hand, have the opposite pattern. This is because their main role in life is not to find food but to defend territory. In years when food is scarce, so many of their fellow tit birds die of hunger that there's enough space for all. The hawkish males then fall into the same trap as their female comrades during nutty seasons—they brawl, squandering precious resources with each bloody battle. But in good years, when competition for nesting territory heats up, aggression pays for the hawkish male tit bird.

During times of war or fear—the human equivalent of a bad nut season for female tit birds—it might seem that what we need most are aggressive heroic types. But if our entire population consisted of warriors, there would be no one to notice, let alone battle, potentially deadly but far quieter threats like viral disease or climate change.

Consider Vice President Al Gore's decades-long crusade to raise awareness of global warming. Gore is, by many accounts, an introvert. "If you send an introvert into a reception or an event with a hundred other people he will emerge with less energy than he had going in," says a former aide. "Gore needs a rest after an event." Gore acknowledges that his skills are not conducive to stumping and speechmaking. "Most people in politics draw energy from backslapping and shaking hands and all that," he has said. "I draw energy from discussing ideas."

But combine that passion for thought with attention to subtlety—both common characteristics of introverts—and you get a very powerful mix. In 1968, when Gore was a college student at Harvard, he took a class with an influential oceanographer who presented early evidence linking the burning of fossil fuels with the greenhouse effect. Gore's ears perked up.

He tried to tell others what he knew. But he found that people wouldn't listen. It was as if they couldn't hear the alarm bells that rang so loudly in his ears.

"When I went to Congress in the middle of the 1970s, I helped organize the first hearings on global warming," he recalls in the Oscar-winning movie An Inconvenient Truth—a film whose most stirring action scenes involve the solitary figure of Gore wheeling his suitcase through a midnight airport. Gore seems genuinely puzzled that no one paid attention: "I actually thought and believed that the story would be compelling enough to cause a real sea change in the way Congress reacted to that issue. I thought they would be startled, too. And they weren't."

But if Gore had known then what we know now about Kagan's research, and Aron's, he might have been less surprised by his colleagues' reactions. He might even have used his insight into personality psychology to get them to listen. Congress, he could have safely assumed, is made up of some of the least sensitive people in the country—people who, if they'd been kids in one of Kagan's experiments, would have marched up to oddly attired clowns and strange ladies wearing gas masks without so much as a backward glance at their mothers. Remember Kagan's introverted Tom and extroverted Ralph? Well, Congress is full of Ralphs—it was designed for people like Ralph. Most of the Toms of the world do not want to spend their days planning campaigns and schmoozing with lobbyists.

These Ralph-like Congressmen can be wonderful people—exuberant, fearless, persuasive—but they're unlikely to feel alarmed by a photograph of a tiny crack in a distant glacier. They need more intense stimulation to get them to listen. Which is why Gore finally got his message across when he teamed up with whiz-bang Hollywood types who could package his warning into the special-effects-laden show that became An Inconvenient Truth.

Gore also drew on his own strengths, using his natural focus and diligence to tirelessly promote the movie. He visited dozens of movie theaters across the country to meet with viewers, and gave innumerable TV and radio interviews. On the subject of global warming, Gore has a clarity of voice that eluded him as a politician. For Gore, immersing himself in a complicated scientific puzzle comes naturally. Focusing on a single passion rather than tap dancing from subject to subject comes naturally. Even talking to crowds comes naturally when the topic is climate change: Gore on global warming has an easy charisma and connection with audience members that eluded him as a political candidate. That's because this mission, for him, is not about politics or personality. It's about the call of his conscience. "It's about the survival of the planet," he says. "Nobody is going to care who won or lost any election when the earth is uninhabitable."

If you're a sensitive sort, then you may be in the habit of pretending to be more of a politician and less cautious or single-mindedly focused than you actually are. But in this chapter I'm asking you to rethink this view. Without people like you, we will, quite literally, drown.

Back here at Walker Creek Ranch and the gathering for sensitive people, the Extrovert Ideal and its primacy of cool is turned upside down. If "cool" is low reactivity that predisposes a person to boldness or nonchalance, then the crowd that has come to meet Elaine Aron is deeply uncool.

The atmosphere is startling simply because it's so unusual. It's something you might find at a yoga class or in a Buddhist monastery, except that here there's no unifying religion or worldview, only a shared temperament. It's easy to see this when Aron delivers her speech. She has long observed that when she speaks to groups of highly sensitive people the room is more hushed and respectful than would be usual in a public gathering place, and this is true throughout her presentation. But it carries over all weekend.

I've never heard so many "after you's" and "thank you's" as I do here. During meals, which are held at long communal tables in a summer-camp style, open-air cafeteria, people plunge hungrily into searching conversations. There's a lot of one-on-one discussion about intimate topics like childhood experiences and adult love lives, and social issues like health care and climate change; there's not much in the way of storytelling intended to entertain. People listen carefully to each other and respond thoughtfully; Aron has noted that sensitive people tend to speak softly because that's how they prefer others to communicate with them.

"In the rest of the world," observes Michelle, a web designer who leans forward as if bracing herself against an imaginary blast of wind, "you make a statement and people may or may not discuss it. Here you make a statement and someone says, 'What does that mean?' And if you ask that question of someone else, they actually answer."

It's not that there's no small talk, observes Strickland, the leader of the gathering. It's that it comes not at the beginning of conversations but at the end. In most settings, people use small talk as a way of relaxing into a new relationship, and only once they're comfortable do they connect more seriously. Sensitive people seem to do the reverse. They "enjoy small talk only after they've gone deep," says Strickland. "When sensitive people are in environments that nurture their authenticity, they laugh and chitchat just as much as anyone else."

On the first night we drift to our bedrooms, housed in a dormlike building. I brace myself instinctively: now's the time when I'll want to read or sleep, but will instead be called upon to have a pillow fight (summer camp) or play a loud and boring drinking game (college). But at Walker Creek Ranch, my roommate, a twenty-seven-year-old secretary with huge, doe-like eyes and the ambition to become an author, is happy to spend the evening writing peacefully in her journal. I do the same.

Of course, the weekend is not completely without tension. Some people are reserved to the point of appearing sullen. Sometimes the do-your-own-thing policy threatens to devolve into mutual loneliness as everyone goes their own separate ways. In fact, there is such a deficit of the social behavior we call "cool" that I begin thinking someone should be cracking jokes, stirring things up, handing out rum-and-Cokes. Shouldn't they?

The truth is, as much as I crave breathing room for sensitive types, I enjoy hail-fellows-well-met, too. I'm glad for the "cool" among us, and I miss them this weekend. I'm starting to speak so softly that I feel like I'm putting myself to sleep. I wonder if deep down the others feel this way, too.

Tom, the software engineer and Abraham Lincoln look-alike, tells me of a former girlfriend who was always throwing open the doors of her house to friends and strangers. She was adventurous in every way: she loved new food, new sexual experiences, new people. It didn't work out between them—Tom eventually craved the company of a partner who would focus more on their relationship and less on the outside world, and he's happily married now to just such a woman—but he's glad for the time with his ex-girlfriend.

As Tom talks, I think of how much I miss my husband, Ken, who's back home in New York and not a sensitive type either, far from it. Sometimes this is frustrating: if something moves me to tears of empathy or anxiety, he'll be touched, but grow impatient if I stay that way too long. But I also know that his tougher attitude is good for me, and I find his company endlessly delightful. I love his effortless charm. I love that he never runs out of interesting things to say. I love how he pours his heart and soul into everything he does, and everyone he loves, especially our family.

But most of all I love his way of expressing compassion. Ken may be aggressive, more aggressive in a week than I'll be in a lifetime, but he uses it on behalf of others. Before we met, he worked for the UN in war zones all over the world, where, among other things, he conducted prisoner-of-war and detainee release negotiations. He would march into fetid jails and face down camp commanders with machine guns strapped to their chests until they agreed to release young girls who'd committed no crime other than to be female and victims of rape. After many years on the job, he went home and wrote down what he'd witnessed, in books and articles that bristled with rage. He didn't write in the style of a sensitive person, and he made a lot of people angry. But he wrote like a person who cares, desperately.

I thought that Walker Creek Ranch would make me long for a world of the highly sensitive, a world in which everyone speaks softly and no one carries a big stick. But instead it reinforced my deeper yearning for balance. This balance, I think, is what Elaine Aron would say is our natural state of being, at least in Indo-European cultures like ours, which she observes have long been divided into "warrior kings" and "priestly advisers," into the executive branch and the judicial branch, into bold and easy FDRs and sensitive, conscientious Eleanor Roosevelts.

7

WHY DID WALL STREET CRASH AND WARREN BUFFETT PROSPER?

How Introverts and Extroverts Think (and Process Dopamine) Differently

Tocqueville saw that the life of constant action and decision which was entailed by the democratic and businesslike character of American life put a premium upon rough and ready habits of mind, quick decision, and the prompt seizure of opportunities—and that all this activity was not propitious for deliberation, elaboration, or precision in thought.

—RICHARD HOFSTADTER, IN Anti-Intellectualism in America

Just after 7:30 a.m. on December 11, 2008, the year of the great stock market crash, Dr. Janice Dorn's phone rang. The markets had opened on the East Coast to another session of carnage. Housing prices were plummeting, credit markets were frozen, and GM teetered on the brink of bankruptcy.

Dorn took the call from her bedroom, as she often does, wearing a headset and perched atop her green duvet. The room was decorated sparely. The most colorful thing in it was Dorn herself, who, with her flowing red hair, ivory skin, and trim frame, looks like a mature version of Lady Godiva. Dorn has a PhD in neuroscience, with a specialty in brain anatomy. She's also an MD trained in psychiatry, an active trader in the gold futures market, and a "financial psychiatrist" who has counseled an estimated six hundred traders.

"Hi, Janice!" said the caller that morning, a confident-sounding man named Alan. "Do you have time to talk?"

Dr. Dorn did not have time. A day trader who prides herself on being in and out of trading positions every half hour, she was eager to start trading. But Dorn heard a desperate note in Alan's voice. She agreed to take the call.

Alan was a sixty-year-old midwesterner who struck Dorn as a salt-of-the-earth type, hardworking and loyal. He had the jovial and assertive manner of an extrovert, and he maintained his good cheer despite the story of disaster he proceeded to tell. Alan and his wife had worked all their lives, and managed to sock away a million dollars for retirement. But four months earlier he'd gotten the idea that, despite having no experience in the markets, he should buy a hundred thousand dollars' worth of GM stock, based on reports that the U.S. government might bail out the auto industry. He was convinced it was a no-lose investment.

After his trade went through, the media reported that the bailout might not happen after all. The market sold off GM and the stock price fell. But Alan imagined the thrill of winning big. It felt so real he could taste it. He held firm. The stock fell again, and again, and kept dropping until finally Alan decided to sell, at a big loss.

There was worse to come. When the next news cycle suggested that the bailout would happen after all, Alan got excited all over again and invested another hundred thousand dollars, buying more stock at the lower price. But the same thing happened: the bailout started looking uncertain.

Alan "reasoned" (this word is in quotation marks because, according to Dorn, conscious reasoning had little to do with Alan's behavior) that the price couldn't go much lower. He held on, savoring the idea of how much fun he and his wife would have spending all the money he stood to make. Again the stock went lower. When finally it hit seven dollars per share, Alan sold. And bought yet again, in a flush of exhilaration, when he heard that the bailout might happen after all ...

By the time GM's stock price fell to two dollars a share, Alan had lost seven hundred thousand dollars, or 70 percent of his family nest egg.

He was devastated. He asked Dorn if she could help recoup his losses. She could not. "It's gone," she told him. "You are never going to make that money back."

He asked what he'd done wrong.

Dorn had many ideas about that. As an amateur, Alan shouldn't have been trading in the first place. And he'd risked far too much money; he should have limited his exposure to 5 percent of his net worth, or $50,000. But the biggest problem may have been beyond Alan's control: Dorn believed he was experiencing an excess of something psychologists call reward sensitivity.

A reward-sensitive person is highly motivated to seek rewards—from a promotion to a lottery jackpot to an enjoyable evening out with friends. Reward sensitivity motivates us to pursue goals like sex and money, social status and influence. It prompts us to climb ladders and reach for faraway branches in order to gather life's choicest fruits.

But sometimes we're too sensitive to rewards. Reward sensitivity on overdrive gets people into all kinds of trouble. We can get so excited by the prospect of juicy prizes, like winning big in the stock market, that we take outsized risks and ignore obvious warning signals.

Alan was presented with plenty of these signals, but was so animated by the prospect of winning big that he couldn't see them. Indeed, he fell into a classic pattern of reward sensitivity run amok: at exactly the moments when the warning signs suggested slowing down, he sped up—dumping money he couldn't afford to lose into a speculative series of trades.

Financial history is full of examples of players accelerating when they should be braking. Behavioral economists have long observed that executives buying companies can get so excited about beating out their competitors that they ignore signs that they're overpaying. This happens so frequently that it has a name: "deal fever," followed by "the winner's curse." The AOL–Time Warner merger, which wiped out $200 billion of Time Warner shareholder value, is a classic example. There were plenty of warnings that AOL's stock, which was the currency for the merger, was wildly overvalued, yet Time Warner's directors approved the deal unanimously.

"I did it with as much or more excitement and enthusiasm as I did when I first made love some forty-two years ago," exclaimed Ted Turner, one of those directors and the largest individual shareholder in the company. "TED TURNER: IT'S BETTER THAN SEX," announced the New York Post the day after the deal was struck, a headline to which we'll return for its power to explain why smart people can sometimes be too reward-sensitive.

You may be wondering what all this has to do with introversion and extroversion. Don't we all get a little carried away sometimes?

The answer is yes, except that some of us do so more than others. Dorn has observed that her extroverted clients are more likely to be highly reward-sensitive, while the introverts are more likely to pay attention to warning signals. They're more successful at regulating their feelings of desire or excitement. They protect themselves better from the downside. "My introvert traders are much more able to say, 'OK, Janice, I do feel these excited emotions coming up in me, but I understand that I can't act on them.' The introverts are much better at making a plan, staying with a plan, being very disciplined."

To understand why introverts and extroverts might react differently to the prospect of rewards, says Dorn, you have to know a little about brain structure. As we saw in chapter 4, our limbic system, which we share with the most primitive mammals and which Dorn calls the "old brain," is emotional and instinctive. It comprises various structures, including the amygdala, and it's highly interconnected with the nucleus accumbens, sometimes called the brain's "pleasure center." We examined the anxious side of the old brain when we explored the role of the amygdala in high reactivity and introversion. Now we're about to see its greedy side.

The old brain, according to Dorn, is constantly telling us, "Yes, yes, yes! Eat more, drink more, have more sex, take lots of risk, go for all the gusto you can get, and above all, do not think!" The reward-seeking, pleasure-loving part of the old brain is what Dorn believes spurred Alan to treat his life savings like chips at the casino.

We also have a "new brain" called the neocortex, which evolved many thousands of years after the limbic system. The new brain is responsible for thinking, planning, language, and decision-making—some of the very faculties that make us human. Although the new brain also plays a significant role in our emotional lives, it's the seat of rationality. Its job, according to Dorn, includes saying, "No, no, no! Don't do that, because it's dangerous, makes no sense, and is not in your best interests, or those of your family, or of society."

So where was Alan's neocortex when he was chasing stock market gains?

The old brain and the new brain do work together, but not always efficiently. Sometimes they're actually in conflict, and then our decisions are a function of which one is sending out stronger signals. So when Alan's old brain sent its breathless messages up to his new brain, it probably responded as a neocortex should: it told his old brain to slow down. It said, Watch out! But it lost the ensuing tug-of-war.

We all have old brains, of course. But just as the amygdala of a high-reactive person is more sensitive than average to novelty, so do extroverts seem to be more susceptible than introverts to the reward-seeking cravings of the old brain. In fact, some scientists are starting to explore the idea that reward-sensitivity is not only an interesting feature of extroversion; it is what makes an extrovert an extrovert. Extroverts, in other words, are characterized by their tendency to seek rewards, from top dog status to sexual highs to cold cash. They've been found to have greater economic, political, and hedonistic ambitions than introverts; even their sociability is a function of reward-sensitivity, according to this view—extroverts socialize because human connection is inherently gratifying.

What underlies all this reward-seeking? The key seems to be positive emotion. Extroverts tend to experience more pleasure and excitement than introverts do—emotions that are activated, explains the psychologist Daniel Nettle in his illuminating book on personality, "in response to the pursuit or capture of some resource that is valued. Excitement builds towards the anticipated capture of that resource. Joy follows its capture." Extroverts, in other words, often find themselves in an emotional state we might call "buzz"—a rush of energized, enthusiastic feelings. This is a sensation we all know and like, but not necessarily to the same degree or with the same frequency: extroverts seem to get an extra buzz from the pursuit and attainment of their goals.

The basis of buzz appears to be a high degree of activity in a network of structures in the brain—often called the "reward system"—including the orbitofrontal cortex, the nucleus accumbens, and the amygdala. The job of the reward system is to get us excited about potential goodies; fMRI experiments have shown that the system is activated by any number of possible delights, from anticipation of a squirt of Kool-Aid on the tongue, to money, to pictures of attractive people.

The neurons that transmit information in the reward network operate in part through a neurotransmitter—a chemical that carries information between brain cells—called dopamine. Dopamine is the "reward chemical" released in response to anticipated pleasures. The more responsive your brain is to dopamine, or the higher the level of dopamine you have available to release, some scientists believe, the more likely you are to go after rewards like sex, chocolate, money, and status. Stimulating mid-brain dopamine activity in mice gets them to run around excitedly in an empty cage until they drop dead of starvation. Cocaine and heroin, which stimulate dopamine-releasing neurons in humans, make people euphoric.

Extroverts' dopamine pathways appear to be more active than those of introverts. Although the exact relationship between extroversion, dopamine, and the brain's reward system has not been conclusively established, early findings have been intriguing. In one experiment, Richard Depue, a neurobiologist at Cornell University, gave an amphetamine that activates the dopamine system to a group of introverts and extroverts, and found that the extroverts had a stronger response. Another study found that extroverts who win gambling games have more activity in the reward-sensitive regions of their brains than victorious introverts do. Still other research has shown that the medial orbitofrontal cortex, a key component of the brain's dopamine-driven reward system, is larger in extroverts than in introverts.

By contrast, introverts "have a smaller response" in the reward system, writes psychologist Nettle, "and so go less out of their way to follow up [reward] cues." They will, "like anyone, be drawn from time to time to sex, and parties, and status, but the kick they get will be relatively small, so they are not going to break a leg to get there." In short, introverts just don't buzz as easily.