The science of charity

Not that Homo sapiens is the only species in which individuals bestow kindness on others. Many mammals, birds, insects and even bacteria do likewise. But their largesse tends to be reserved for their genetic relatives; this makes sense in evolutionary terms, because by helping someone who shares many of your genes you improve the chances of propelling this common DNA into the future. Humans are different, for we cooperate with complete genetic strangers - workmates, neighbours, anonymous people in far-off countries. Why on earth do we do that?