Here is a moral dilemma: Would you be happy to live in a world in which 80 percent of the population enjoys more or less peaceful conditions, but the remaining 20 percent are condemned to live with a worsening spiral of war and suffering?

This is a useful question, because it is a rough description of the actual world we live in. Most of the planet is pretty stable these days. Last week, the cognitive scientist Stephen Pinker and Colombian President Juan Manuel Santos underscored this point in an opinion piece celebrating Colombia’s peace deal with the leftist Revolutionary Armed Forces of Colombia, or FARC. ...