Valentine's concept of introversion includes traits that contemporary psychology would classify as openness to experience ("thinker, dreamer"), conscientiousness ("idealist"), and neuroticism ("shy individual").

A long line of poets, scientists, and philosophers have also tended to group these traits together. All the way back in Genesis, the earliest book of the Bible, we had cerebral Jacob (a "quiet man dwelling in tents" who later becomes "Israel," meaning one who wrestles inwardly with God) squaring off in sibling rivalry with his brother, the swashbuckling Esau (a "skillful hunter" and "man of the field"). In classical antiquity, the physicians Hippocrates and Galen famously proposed that our temperaments—and destinies—were a function of our bodily fluids, with extra blood and "yellow bile" making us sanguine or choleric (stable or neurotic extroversion), and an excess of phlegm and "black bile" making us calm or melancholic (stable or neurotic introversion). Aristotle noted that the melancholic temperament was associated with eminence in philosophy, poetry, and the arts (today we might classify this as openness to experience). The seventeenth-century English poet John Milton wrote Il Penseroso ("The Thinker") and L'Allegro ("The Merry One"), comparing "the happy person" who frolics in the countryside and revels in the city with "the thoughtful person" who walks meditatively through the nighttime woods and studies in a "lonely Towr." (Again, today the description of Il Penseroso would apply not only to introversion but also to openness to experience and neuroticism.) The nineteenth-century German philosopher Schopenhauer contrasted "good-spirited" people (energetic, active, and easily bored) with his preferred type, "intelligent people" (sensitive, imaginative, and melancholic). "Mark this well, ye proud men of action!" declared his countryman Heinrich Heine. "Ye are, after all, nothing but unconscious instruments of the men of thought."

Because of this definitional complexity, I originally planned to invent my own terms for these constellations of traits. I decided against this, again for cultural reasons: the words introvert and extrovert have the advantage of being well known and highly evocative. Every time I uttered them at a dinner party or to a seatmate on an airplane, they elicited a torrent of confessions and reflections. For similar reasons, I've used the layperson's spelling of extrovert rather than the extravert one finds throughout the research literature.

Acknowledgments

I could not have written Quiet without the help of countless friends, family members, and colleagues, including: Richard Pine, otherwise known (to me) as Super-Agent RSP: the smartest, savviest, and menschiest literary agent that any writer could hope to work with. Richard believed unswervingly in Quiet, even before I did. Then he kept on believing, all the way through the five years it took me to research and write it. I consider him not only an agent but a partner in my career. I also enjoyed working with the whole team at InkWell Management, including Ethan Bassoff, Lyndsey Blessing, and Charlie Olsen.

At Crown Publishers, it has been my privilege to work with the remarkable Molly Stern and her all-star team. Rachel Klayman has got to be the most brilliant and dedicated editor in the business. She has been there at two in the afternoon and at two in the morning, spotting flaws in my reasoning and clunkers in my prose, and championing this book indefatigably. I also appreciate how generous Mary Choteborsky and Jenna Ciongoli were with their editorial talents. And I was fortunate to work with outside editor Peter Guzzardi, who has terrific instincts and a knack for making criticism sound delightful. My heartfelt thanks to all of you. This book would be a shadow of itself without your efforts.

Special thanks too to Rachel Rokicki and Julie Cepler for the creativity and enthusiasm they brought to the Quiet cause. And thanks to Patty Berg, Mark Birkey, Chris Brand, Stephanie Chan, Tina Constable, Laura Duffy, Songhee Kim, Kyle Kolker, Rachel Meier, Annsley Rosner, and everyone else on the team at Crown.

I have also been very lucky to work with Joel Rickett, Kate Barker, and the rest of the crackerjack group at Viking/Penguin U.K.

The marvelous people at TED embraced the ideas in this book and offered me a chance to talk about them at the TED Long Beach conference in 2012. I am grateful to Chris Anderson, Kelly Stoetzel, June Cohen, Tom Rielly, Michael Glass, Nicholas Weinberg, and the entire TED team.

Brian Little, whose work I profiled in chapter 9, has become an extraordinary mentor and friend. I met Brian early in my research process, when I asked for an interview. He gave me not only the interview but also, over the years, my own personal graduate seminar in personality psychology. I am proud to be one of his many disciples and friends.

Elaine Aron, whose research I profiled in chapter 6, inspired me with her life's work and gave generously of her time, knowledge, and life story.

I relied on the support and advice of innumerable friends, including: Marci Alboher, Gina Bianchini, Tara Bracco, Janis Brody, Greg Bylinksy, David Callahan, Helen Churko, Mark Colodny, Estie Dallett, Ben Dattner, Ben Falchuk, Christy Fletcher, Margo Flug, Jennifer Gandin Le, Rhonda Garelick, Michael Glass, Vishwa Goohya, Leeat Granek, Amy Gutman, Hillary Hazan-Glass, Wende Jaeger-Hyman, Mahima Joishy, Emily Klein, Chris Le, Rachel Lehmann-Haupt, Lori Lesser, Margot Magowan, Courtney Martin, Fran and Jerry Marton, Furaha Norton, Elizabeth O'Neill, Wendy Paris, Leanne Paluck Reiss, Marta Renzi, Gina Rudan, Howard Sackstein, Marisol Simard, Daphna Stern, Robin Stern, Tim Stock, Jillian Straus, Sam Sugiura, Tom Sugiura, Jennifer Taub, Kate Tedesco, Ruti Teitel, Seinenu Thein, Jacquette Timmons, Marie Lena Tupot, Sam Walker, Daniel Wolff, and Cali Yost. A special, super-duper thanks to Anna Beltran, Maritza Flores, and Eliza Simpson.

I am especially grateful for the forbearance of some of my oldest and dearest friends: Mark Colodny, Jeff Kaplan, Hitomi Komatsu, Cathy Lankenau-Weeks, Lawrence Mendenhall, Jonathan Sichel, Brande Stellings, Judith van der Reis, Rebecca and Jeremy Wallace-Segall, and Naomi Wolf, who remain close even though we barely had time to talk, let alone visit, during the years I wrote this book and gave birth to my two children.

Thank you, too, to my fellow members of the Invisible Institute, who inspire and astonish me on a regular basis: Gary Bass, Elizabeth Devita-Raeburn, Abby Ellin, Randi Epstein, Sheri Fink, Christine Kenneally, Judith Matloff, Katie Orenstein, Annie Murphy Paul, Pamela Paul, Joshua Prager, Alissa Quart, Paul Raeburn, Kathy Rich, Gretchen Rubin, Lauren Sandler, Deborah Siegel, Rebecca Skloot, Debbie Stier, Stacy Sullivan, Maia Szalavitz, Harriet Washington, and Tom Zoellner.

For inspiration that I would bottle and sell if I could, I thank the owners of the cottage in Amagansett: Alison (Sunny) Warriner and Jeanne Mclemore. The same goes for Evelyn and Michael Polesny, proprietors of the magical Doma Café in Greenwich Village, where I wrote most of this book.

Thanks also to those who helped with various aspects of getting Quiet off the ground: Nancy Ancowitz, Mark Colodny, Bill Cunningham, Ben Dattner, Aaron Fedor, Boris Fishman, David Gallo, Christopher Glazek, Suzy Hansen, Jayme Johnson, Jennifer Kahweiler, David Lavin, Ko-Shin Mandell, Andres Richner, JillEllyn Riley, Gretchen Rubin, Gregory Samanez-Larkin, Stephen Schueller, Sree Sreenivasan, Robert Stelmack, Linda Stone, John Thompson, Charles Yao, Helen Wan, Georgia Weinberg, and Naomi Wolf.

I owe a special debt to the people I wrote about or quoted, some of whom have become friends: Michel Anteby, Jay Belsky, Jon Berghoff, Wayne Cascio, Hung Wei Chien, Boykin Curry, Tom DeMarco, Richard Depue, Dr. Janice Dorn, Anders Ericsson, Jason Fried, Francesca Gino, Adam Grant, William Graziano, Stephen Harvill, David Hofmann, Richard Howard, Jadzia Jagiellowicz, Roger Johnson, Jerry Kagan, Guy Kawasaki, Camelia Kuhnen, Tiffany Liao, Richard Lippa, Joanna Lipper, Adam McHugh, Mike Mika, Emily Miller, Jerry Miller, Quinn Mills, Purvi Modi, Joseph Newman, Preston Ni, Carl Schwartz, Dave Smith, Mark Snyder, Jacqueline Strickland, Avril Thorne, David Weiss, Mike Wei, and Shoya Zichy.

There are many, many others who aren't mentioned by name in Quiet but who gave generously of their time and wisdom, via interviews and the like, and who dramatically informed my thinking: Marco Acevedo, Anna Allanbrook, Andrew Ayre, Dawn Rivers Baker, Susan Blew, Jonathan Cheek, Jeremy Chua, Dave Coleman, Ben Dattner, Matthew Davis, Scott Derue, Carl Elliott, Brad Feld, Kurt Fischer, Alex Forbes, Donna Genyk, Carole Grand, Stephen Gerras, Lenny Gucciardi, Anne Harrington, Naomi Karten, James McElroy, Richard McNally, Greg Oldham, Christopher Peterson, Lise Quintana, Lena Roy, Chris Scherpenseel, Hersh Shefrin, Nancy Snidman, Sandy Tinkler, Virginia Vitzthum, E. O. Wilson, David Winter, and Patti Wollman. Thank you, all.

Most of all I thank my family: Lawrence and Gail Horowitz, Barbara Schnipper, and Mitchell Horowitz, whom I wrote about in the dedication; Lois, Murray, and Steve Schnipper, who make the world a warmer place; Steve and Gina Cain, my wonderful West Coast siblings; and the inimitable Heidi Postlewait.

Special thanks and love to Al and Bobbi Cain, who lent me their advice, contacts, and professional counsel as I researched and wrote, and who constantly cause me to hope that one day I will be as devoted and supportive an in-law to some young person as they are to me.

And to my beloved Gonzo (a.k.a. Ken), who may just be the most generous person on earth, and the most dashing. During the years I wrote this book, he edited my drafts, sharpened my ideas, made me tea, made me laugh, brought me chocolate, seeded our garden, turned his world upside down so I had time to write, kept our lives colorful and exciting, and got us the hell out of the Berkshires. He also, of course, gave us Sammy and Elishku, who have filled our house with trucks and our hearts with love.