About?” My squib provoked a flurry of correspondence from scholars in historical criminology and international studies who told me that the evidence for a historical reduction in violence is more extensive than I had realized.4 It was their data that convinced me that there was an underappreciated story waiting to be told.
My first and deepest thanks go to these scholars: Azar Gat, Joshua Goldstein, Manuel Eisner, Andrew Mack, John Mueller, and John Carter Wood. As I worked on the book, I also benefited from correspondence with Peter Brecke, Tara Cooper, Jack Levy, James Payne, and Randolph Roth. These generous researchers shared ideas, writings, and data and kindly guided me through fields of research that are far from my own specialization.
David Buss, Martin Daly, Rebecca Newberger Goldstein, David Haig, James Payne, Roslyn Pinker, Jennifer Sheehy-Skeffington, and Polly Wiessner read most or all of the first draft and offered immeasurably helpful advice and criticism. Also invaluable were comments on particular chapters offered by Peter Brecke, Daniel Chirot, Alan Fiske, Jonathan Gottschall, A. C. Grayling, Niall Ferguson, Graeme Garrard, Joshua Goldstein, Capt. Jack Hoban, Stephen Leblanc, Jack Levy, Andrew Mack, John Mueller, Charles Seife, Jim Sidanius, Michael Spagat, Richard Wrangham, and John Carter Wood.
Many other people responded to my inquiries with prompt explanations or offered suggestions that were incorporated into the book: John Archer, Scott Atran, Daniel Batson, Donald Brown, Lars-Erik Cederman, Christopher Chabris, Gregory Cochran, Leda Cosmides, Tove Dahl, Lloyd deMause, Jane Esberg, Alan Fiske, Dan Gardner, Pinchas Goldschmidt, Cmdr. Keith Gordon, Reid Hastie, Brian Hayes, Judith Rich Harris, Harold Herzog, Fabio Idrobo, Tom Jones, Maria Konnikova, Robert Kurzban, Gary Lafree, Tom Lehrer, Michael Macy, Steven Malby, Megan Marshall, Michael McCullough, Nathan Myhrvold, Mark Newman, Barbara Oakley, Robert Pinker, Susan Pinker, Ziad Obermeyer, David Pizarro, Tage Rai, David Ropeik, Bruce Russett, Scott Sagan, Ned Sahin, Aubrey Sheiham, Francis X. Shen, Lt. Col. Joseph Shusko, Richard Shweder, Thomas Sowell, Håvard Strand, Ilavenil Subbiah, Rebecca Sutherland, Philip Tetlock, Andreas Forø Tollefsen, James Tucker, Staffan Ulfstrand, Jeffrey Watumull, Robert Whiston, Matthew White, Maj. Michael Wiesenfeld, and David Wolpe.
Many colleagues and students at Harvard have been generous with their expertise, including Mahzarin Banaji, Robert Darnton, Alan Dershowitz, James Engell, Nancy Etcoff, Drew Faust, Benjamin Friedman, Daniel Gilbert, Edward Glaeser, Omar Sultan Haque, Marc Hauser, James Lee, Bay McCulloch, Richard McNally, Michael Mitzenmacher, Orlando Patterson, Leah Price, David Rand, Robert Sampson, Steve Shavell, Lawrence Summers, Kyle Thomas, Justin Vincent, Felix Warneken, and Daniel Wegner.
Special thanks go to the researchers who have worked with me on the data reported in these pages. Brian Atwood carried out countless statistical analyses and database searches with precision, thoroughness, and insight. William Kowalsky discovered many pertinent findings from the world of public opinion polling. Jean-Baptiste Michel helped develop the Bookworm program, the Google Ngram Viewer, and the Google Books corpus and devised an ingenious model for the distribution of the magnitude of wars. Bennett Haselton carried out an informative study of people’s perceptions of the history of violence. Esther Snyder assisted with graphing and bibliographic searches. Ilavenil Subbiah designed the elegant graphs and maps, and over the years has provided me with invaluable insight about the culture and history of Asia.
John Brockman, my literary agent, posed the question that led to the writing of this book and offered many helpful comments on the first draft. Wendy Wolf, my editor at Penguin, offered a detailed analysis of the first draft that did much to shape the final version. I’m enormously grateful to John and Wendy, together with Will Goodlad at Penguin UK, for their support of the book at every stage.
Heartfelt thanks go to my family for their love and encouragement: Harry, Roslyn, Susan, Martin, Robert, and Kris. My greatest appreciation goes to Rebecca Newberger Goldstein, who not only improved the book’s substance and style but encouraged me with her belief in the value of the project, and who has done more than anyone to shape my worldview. This book is dedicated to my niece, nephews, and stepdaughters: may they enjoy a world in which the decline of violence continues.
1
A FOREIGN COUNTRY
The past is a foreign country: they do things differently there.
—L. P. Hartley
If the past is a foreign country, it is a shockingly violent one. It is easy to forget how dangerous life used to be, how deeply brutality was once woven into the fabric of daily existence. Cultural memory pacifies the past, leaving us with pale souvenirs whose bloody origins have been bleached away. A woman donning a cross seldom reflects