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 "