You have 2,000 friends on Facebook, but who would bring you soup if you were sick? Evolutionary psychologist Sarah Penn explores "Dunbar's Number" in "The Tribe of 150." Based on the research of Robin Dunbar, Penn explains that the human neocortex is only capable of maintaining stable relationships with about 150 people.The book breaks down the circles of intimacy: the 5 loved ones, the 15 good friends, the 50 acquaintances, and the 150 tribe members. Penn argues that social media attempts to hack this biological limit, creating a "social snacking" effect that leaves us feeling full but...