Human beings can be so compassionate. They can also be shockingly cruel. What if there was a master control for human behaviour? Switch it on and people are loving and generous. Switch it off and they revert to violence and greed. Pioneering neuroeconomist Paul J. Zak has discovered just such a master switch: a molecule in the human brain.

Zak's colleagues call him Dr Love. They also call him the vampire economist. He and his research team have travelled from his laboratory in California to the jungles of Papua new Guinea via a summer garden in Devon, taking blood from people as they attend a wedding, make decisions with money, play football on the field, even jump from an aeroplane. Their experiments to measure a chemical in the bloodstream called oxytocin reveal the answers to those mysteries about why we make the decisions we do: why we are sometimes rational, at other times irrational; why men cheat more than women; how the moral molecule operates in the market place, and most importantly, once we understand the moral molecule, how we can consciously use it to make our lives better.