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.

Robin Dunbar 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 Friends, he looks at friendship in the round, at the way different types of friendship and family relationships intersect, or at 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.

Mixing insights from scientific research with first person experiences and culture, Friends explores and integrates knowledge from disciplines ranging from psychology and anthropology to neuroscience and genetics in a single magical weave that allows us to peer into the incredible complexity of the social world in which we are all so deeply embedded.

Working at the coalface of the subject at both research and personal levels, Robin Dunbar has written the definitive book on how and why we are friends.