Robin Dunbar is a world renowned professor of evolutionary biology. He explains his famous theory of social group sizes, known as Dunbar's number, which suggests there is a limited number of relationships which humans (and other primates) can maintain. We also discuss some of his other findings related to social bonding and the release of endorphins activated by certain interactions such as touch, laughing, singing, and dancing together. Professor Dunbar is with the Department of Experimental Psychology at the University of Oxford and he is a Fellow of Magdalen College.