Friends matter to us, and they matter more than we think. The single most surprising fact to emerge out of the medical literature over the last decade or so has been that the number and quality of the friendships we have has a bigger influence on our happiness, health and even mortality risk than anything else except giving up smoking.

Our guest is the world-renowned psychologist and author who famously discovered Dunbar's number: how our capacity for friendship is limited to around 150 people. In today's book,he explores the way different types of friendship and family relationships intersect, and the complex of psychological and behavioural mechanisms that underpin friendships and make them possible - and just how complicated the business of making and keeping friends actually is.

Working at the coalface of the subject at both research and personal levels, he has written the definitive book on how and why we are friends. We welcome evolutionary psychologist and former director of the Institute of Cognitive and Evolutionary Anthropology and the Department of Experimental Psychology at Oxford University. His acclaimed books include How Many Friends Does One Person Need? Grooming, Gossip and the Evolution of Language and so many more.