Stephen Pinker's optimistic 2011 book argues that, despite humanity's biological tendency toward violence, we are, in fact, less violent today than ever before citing extensive statistical evidence. For him, much of the credit for the decline goes to the eighteenth-century Enlightenment movement, whose ideas of liberty, tolerance, and respect for the value of human life filtered down through society and affected how people thought. That psychological change led to behavioral change—and overall we became more peaceful. Critics countered that humanity could never overcome the biological urge toward violence; others argued that Pinker's statistics were flawed. The debate rages on...