What drives Zak's hunger for human blood is his interest in the hormone oxytocin, about which he has become one of the world's most prominent experts. Long known as a female reproductive hormone - it plays a central role in childbirth and breastfeeding - oxytocin emerges from Zak's research as something much more all-embracing: the "moral molecule" behind all human virtue, trust, affection and love, "a social glue", as he puts it, "that keeps society together". The subtitle of his book, "the new science of what makes us good or evil", gives a sense of the scale of his ambition, which involves nothing less than explaining whole swaths of philosophical and religious questions by reference to a single chemical in the bloodstream.