Today Laura understands that her introversion is an essential part of who she is, and she embraces her reflective nature. The loop inside her head that accused her of being too quiet and una.s.suming plays much less often. Laura knows that she can hold her own when she needs to.

What exactly do I mean when I say that Laura is an introvert? When I started writing this book, the first thing I wanted to find out was precisely how researchers define introversion and extroversion. I knew that in 1921 the influential psychologist Carl Jung had published a bombsh.e.l.l of a book, Psychological Types, popularizing the terms introvert and extrovert as the central building blocks of personality. Introverts are drawn to the inner world of thought and feeling, said Jung, extroverts to the external life of people and activities. Introverts focus on the meaning they make of the events swirling around them; extroverts plunge into the events themselves. Introverts recharge their batteries by being alone; extroverts need to recharge when they don't socialize enough. If you've ever taken a Myers-Briggs personality test, which is based on Jung's thinking and used by the majority of universities and Fortune 100 companies, then you may already be familiar with these ideas.

But what do contemporary researchers have to say? I soon discovered that there is no all-purpose definition of introversion or extroversion; these are not unitary categories, like "curly-haired" or "sixteen-year-old," in which everyone can agree on who qualifies for inclusion. For example, adherents of the Big Five school of personality psychology (which argues that human personality can be boiled down to five primary traits) define introversion not in terms of a rich inner life but as a lack of qualities such as a.s.sertiveness and sociability. There are almost as many definitions of introvert and extrovert as there are personality psychologists, who spend a great deal of time arguing over which meaning is most accurate. Some think that Jung's ideas are outdated; others swear that he's the only one who got it right.

Still, today's psychologists tend to agree on several important points: for example, that introverts and extroverts differ in the level of outside stimulation that they need to function well. Introverts feel "just right" with less stimulation, as when they sip wine with a close friend, solve a crossword puzzle, or read a book. Extroverts enjoy the extra bang that comes from activities like meeting new people, skiing slippery slopes, and cranking up the stereo. "Other people are very arousing," says the personality psychologist David Winter, explaining why your typical introvert would rather spend her vacation reading on the beach than partying on a cruise ship. "They arouse threat, fear, flight, and love. A hundred people are very stimulating compared to a hundred books or a hundred grains of sand."

Many psychologists would also agree that introverts and extroverts work differently. Extroverts tend to tackle a.s.signments quickly. They make fast (sometimes rash) decisions, and are comfortable mult.i.tasking and risk-taking. They enjoy "the thrill of the chase" for rewards like money and status.

Introverts often work more slowly and deliberately. They like to focus on one task at a time and can have mighty powers of concentration. They're relatively immune to the lures of wealth and fame.

Our personalities also shape our social styles. Extroverts are the people who will add life to your dinner party and laugh generously at your jokes. They tend to be a.s.sertive, dominant, and in great need of company. Extroverts think out loud and on their feet; they prefer talking to listening, rarely find themselves at a loss for words, and occasionally blurt out things they never meant to say. They're comfortable with conflict, but not with solitude.

Introverts, in contrast, may have strong social skills and enjoy parties and business meetings, but after a while wish they were home in their pajamas. They prefer to devote their social energies to close friends, colleagues, and family. They listen more than they talk, think before they speak, and often feel as if they express themselves better in writing than in conversation. They tend to dislike conflict. Many have a horror of small talk, but enjoy deep discussions.

A few things introverts are not: The word introvert is not a synonym for hermit or misanthrope. Introverts can be these things, but most are perfectly friendly. One of the most humane phrases in the English language-"Only connect!"-was written by the distinctly introverted E. M. Forster in a novel exploring the question of how to achieve "human love at its height."

Nor are introverts necessarily shy. Shyness is the fear of social disapproval or humiliation, while introversion is a preference for environments that are not overstimulating. Shyness is inherently painful; introversion is not. One reason that people confuse the two concepts is that they sometimes overlap (though psychologists debate to what degree). Some psychologists map the two tendencies on vertical and horizontal axes, with the introvert-extrovert spectrum on the horizontal axis, and the anxious-stable spectrum on the vertical. With this model, you end up with four quadrants of personality types: calm extroverts, anxious (or impulsive) extroverts, calm introverts, and anxious introverts. In other words, you can be a shy extrovert, like Barbra Streisand, who has a larger-than-life personality and paralyzing stage fright; or a non-shy introvert, like Bill Gates, who by all accounts keeps to himself but is unfazed by the opinions of others.

You can also, of course, be both shy and an introvert: T. S. Eliot was a famously private soul who wrote in "The Waste Land" that he could "show you fear in a handful of dust." Many shy people turn inward, partly as a refuge from the socializing that causes them such anxiety. And many introverts are shy, partly as a result of receiving the message that there's something wrong with their preference for reflection, and partly because their physiologies, as we'll see, compel them to withdraw from high-stimulation environments.

But for all their differences, shyness and introversion have in common something profound. The mental state of a shy extrovert sitting quietly in a business meeting may be very different from that of a calm introvert-the shy person is afraid to speak up, while the introvert is simply overstimulated-but to the outside world, the two appear to be the same. This can give both types insight into how our reverence for alpha status blinds us to things that are good and smart and wise. For very different reasons, shy and introverted people might choose to spend their days in behind-the-scenes pursuits like inventing, or researching, or holding the hands of the gravely ill-or in leadership positions they execute with quiet competence. These are not alpha roles, but the people who play them are role models all the same.

If you're still not sure where you fall on the introvert-extrovert spectrum, you can a.s.sess yourself here. Answer each question "true" or "false," choosing the answer that applies to you more often than not.*

1. _______ I prefer one-on-one conversations to group activities.

2. _______ I often prefer to express myself in writing.

3. _______ I enjoy solitude.

4. _______ I seem to care less than my peers about wealth, fame, and status.

5. _______ I dislike small talk, but I enjoy talking in depth about topics that matter to me.

6. _______ People tell me that I'm a good listener.

7. _______ I'm not a big risk-taker.

8. _______ I enjoy work that allows me to "dive in" with few interruptions.

9. _______ I like to celebrate birthdays on a small scale, with only one or two close friends or family members.

10. _______ People describe me as "soft-spoken" or "mellow."

11. _______ I prefer not to show or discuss my work with others until it's finished.

12. _______ I dislike conflict.

13. _______ I do my best work on my own.

14. _______ I tend to think before I speak.

15. _______ I feel drained after being out and about, even if I've enjoyed myself.

16. _______ I often let calls go through to voice mail.

17. _______ If I had to choose, I'd prefer a weekend with absolutely nothing to do to one with too many things scheduled.

18. _______ I don't enjoy mult.i.tasking.

19. _______ I can concentrate easily.

20. _______ In cla.s.sroom situations, I prefer lectures to seminars.

The more often you answered "true," the more introverted you probably are. If you found yourself with a roughly equal number of "true" and "false" answers, then you may be an ambivert-yes, there really is such a word.

But even if you answered every single question as an introvert or extrovert, that doesn't mean that your behavior is predictable across all circ.u.mstances. We can't say that every introvert is a bookworm or every extrovert wears lampshades at parties any more than we can say that every woman is a natural consensus-builder and every man loves contact sports. As Jung felicitously put it, "There is no such thing as a pure extrovert or a pure introvert. Such a man would be in the lunatic asylum."

This is partly because we are all gloriously complex individuals, but also because there are so many different kinds of introverts and extroverts. Introversion and extroversion interact with our other personality traits and personal histories, producing wildly different kinds of people. So if you're an artistic American guy whose father wished you'd try out for the football team like your rough-and-tumble brothers, you'll be a very different kind of introvert from, say, a Finnish businesswoman whose parents were lighthouse keepers. (Finland is a famously introverted nation. Finnish joke: How can you tell if a Finn likes you? He's staring at your shoes instead of his own.)

Many introverts are also "highly sensitive," which sounds poetic, but is actually a technical term in psychology. If you are a sensitive sort, then you're more apt than the average person to feel pleasantly overwhelmed by Beethoven's "Moonlight Sonata" or a well-turned phrase or an act of extraordinary kindness. You may be quicker than others to feel sickened by violence and ugliness, and you likely have a very strong conscience. When you were a child you were probably called "shy," and to this day feel nervous when you're being evaluated, for example when giving a speech or on a first date. Later we'll examine why this seemingly unrelated collection of attributes tends to belong to the same person and why this person is often introverted. (No one knows exactly how many introverts are highly sensitive, but we know that 70 percent of sensitives are introverts, and the other 30 percent tend to report needing a lot of "down time.")

All of this complexity means that not everything you read in Quiet will apply to you, even if you consider yourself a true-blue introvert. For one thing, we'll spend some time talking about shyness and sensitivity, while you might have neither of these traits. That's OK. Take what applies to you, and use the rest to improve your relationships with others.

Having said all this, in Quiet we'll try not to get too hung up on definitions. Strictly defining terms is vital for researchers whose studies depend on pinpointing exactly where introversion stops and other traits, like shyness, start. But in Quiet we'll concern ourselves more with the fruit of that research. Today's psychologists, joined by neuroscientists with their brain-scanning machines, have unearthed illuminating insights that are changing the way we see the world-and ourselves. They are answering questions such as: Why are some people talkative while others measure their words? Why do some people burrow into their work and others organize office birthday parties? Why are some people comfortable wielding authority while others prefer neither to lead nor to be led? Can introverts be leaders? Is our cultural preference for extroversion in the natural order of things, or is it socially determined? From an evolutionary perspective, introversion must have survived as a personality trait for a reason-so what might the reason be? If you're an introvert, should you devote your energies to activities that come naturally, or should you stretch yourself, as Laura did that day at the negotiation table?

The answers might surprise you.

If there is only one insight you take away from this book, though, I hope it's a newfound sense of ent.i.tlement to be yourself. I can vouch personally for the life-transforming effects of this outlook. Remember that first client I told you about, the one I called Laura in order to protect her ident.i.ty?

That was a story about me. I was my own first client.

* Answer key: exercise: extroverts; commit adultery: extroverts; function well without sleep: introverts; learn from our mistakes: introverts; place big bets: extroverts; delay gratification: introverts; be a good leader: in some cases introverts, in other cases extroverts, depending on the type of leadership called for; ask "what if": introverts.

* Sir Isaac Newton, Albert Einstein, W. B. Yeats, Frederic Chopin, Marcel Proust, J. M. Barrie, George Orwell, Theodor Geisel (Dr. Seuss), Charles Schulz, Steven Spielberg, Larry Page, J. K. Rowling.

* This is an informal quiz, not a scientifically validated personality test. The questions were formulated based on characteristics of introversion often accepted by contemporary researchers.