One of the delightful features of our field is that a man who wrote an undergraduate thesis on auditory perception, a doctoral thesis on visual imagery, two technical books on children's acquisition of syntax, and a series of monographs on the psychology of irregular verbs could name as his most important scientific contribution an analysis of the causes of war and peace, crime and safety, barbarism and humanity. This analysis appeared in my 2011 book The Better Angels of Our Nature: Why Violence Has Declined. The subtitle is about history, but the main title is defiantly psychological: it's about the components of human nature that inhibit us from causing physical harm.
It was easy to designate this book (and the papers I have written in its wake), as my most important contribution, because that's what the world has told me. It's been discussed by columnists, politicians, diplomats, generals, even a president and prime minister or two. Given the book's topic and message, this should not have been surprising. I wrote in its preface:
This book is about what may be the most important thing that has ever happened in human history. Believe it or not – and I know that most people do not – violence has declined over long stretches of time, and today we may be living in the most peaceable era in our species's existence …
No aspect of life is untouched by the retreat from violence. Daily existence is very different if you always have to worry about being abducted, raped, or killed, and it's hard to develop sophisticated arts, learning, or commerce if the institutions that support them are looted and burned as quickly as they are built.
The historical trajectory of violence affects not only how life is lived but how it is understood. What could be more fundamental to our sense of meaning and purpose than a conception of whether the strivings of the human race over long stretches of time have left us better or worse off? How, in particular, are we to make sense of modernity – of the erosion of family, tribe, tradition, and religion by the forces of individualism, cosmopolitanism, reason, and science?