Jim talks with evolutionary psychologist Robin Dunbar, discoverer of Dunbar's number, about his latest book, Friends: Understanding the Power of our Most Important Relationships. They cover the importance of friendship, the loneliness epidemic, loneliness as a signal rather than a disease, oxytocin & endorphins, physical touch, synchrony & other ways of triggering the endorphin system, new data sources in the study of social networks, the social brain hypothesis, theory of mind/mind-reading, limitations of our mind-simulating capacities, the discovery of the Dunbar number(s), examples of the pattern from Navy Seals to Christmas cards, the mystery behind the scaling law of three, the seven pillars of friendship, costs & benefits of diversity, why friendships end, and more.