"Do unto others as you would have them do unto you." Everyone, at some point in their life, has probably heard some variation on this old adage. It is present in all the major world religions. As a moral imperative, it is difficult to argue with, even if some philosophers have tried. Perhaps the reason why it seems so unquestionable is that it is the linguistic expression of an ancient survival tactic, one that has been so successful that it has become ingrained in our evolutionary psychology. Kindness to strangers, as Michael E. McCullough explains in his new book, has a very long history, one that is inextricably linked to the endurance of our species.

Initially, a cynic might balk at such a commendatory assessment of the human race. Suspend your judgment. McCullough is social scientist, not a philosopher. Although he writes with the disarming alacrity of a born optimist, The ...