Why do we feel an emotion as potentially catastrophic as love, and what do our fingers have to reveal about our fidelity? Evolutionary psychologist Robin Dunbar breaks down the startling biological reality that humans are essentially 'serial monogamists' by nature, driven by a neurochemical cocktail designed to ensure offspring survival. He reveals the 'show-off hypothesis' behind male risk-taking and explains why the common kiss is actually a sophisticated biological audit of your partner's immune system.