The propensity for identity politics to inject rancour and distrust into public life is a topic that has been widely discussed on both sides of the Atlantic, though rarely from the perspective of the evolution of human nature. However, it is useful to look at it from such a perspective because it can help explain why the passions aroused can so quickly become dangerous, up to and including not just the demonisation of others, but even the murder of those perceived as the enemy. The key elements of what it means to be human, and how we relate to one...