Robin Dunbar is a professor, an evolutionary psychologist, and the author of many books, including "Friends: Understanding the Power of Our Most Important Relationships." During our conversation, Robin talks about how and why he became interested in evolutionary psychology, what evolutionary psychology is, and its explanatory power.

He also talks about human social dynamics, and our "circles of friendship." These concentric circles include the universal findings of how, on average, human numerically structure their social lives: intimate friends (1.5), close friends (5), best friends (15), good friends (50), friends (150), acquaintances (500), and known names (1,500). "Dunbar's number" of 150 is really just one of these tiers.

Finally, Robin discusses why humans have friends, the "seven pillars of friendship," how people vet others for their appropriate tier early in friendship, and the loneliness people often experience when their "inner circles" are not robust and strong. The health and endorphin benefits of real friends, Robin notes, is often better than any therapy or medicine, and is free to all. I loved talking to this fascinating, friendly, and funny scholar, and hope to have him back on the show not too far down the road.