Why do we give a damn about strangers? Altruism is unique to the human species. It is also one of the great evolutionary puzzles, and we may be on the brink of solving it. It turns out that, over the last 12,000 years, we have become more and more altruistic. This is despite the fact that, the majority of the time, our minds are still breathtakingly indifferent to the welfare of others. In solving the enigma of generosity in a world of strangers, McCullough takes us on a sweeping history of society and science to warn that, if we are not careful, our instincts and sympathies have as much potential for harm as for good. The bad news is that we are not designed to be kind. The good news is that we can push ourselves to be kind anyway, together.