Rosie von Lila and Raman Frey take a deep dive into the science of why we categorize ourselves into in-crowds and out-crowds. Explore David Berreby's excellent 2004 book: Us and Them: Understanding Your Tribal Mind. 

Raman Frey is the founder of Good People Dinners -- a San Francisco based community that fosters trust and friendship through food, drink and conversation. Over one hundred-fifty gatherings, dinners, overnights and retreats, have occurred. You can read his work on Medium.com.