On July 23, 2011, Anders Breivik opened fire at a youth camp on the Norwegian island of Utoya, killing 69 people. Breivik’s brutal crime horrified the world. For many Americans, it stirred up memories of Oklahoma City in 1994, Columbine in 1999, and, of course, September 11, 2001. Yet, despite what looks like a proliferation of shooting sprees and terrorist plots in the last few decades, a new book argues that violence has actually declined since ancient times.

Steven Pinker, professor of psychology at Harvard University, is the author of "The Better Angels of Our Nature:Why Violence Has Declined."