It’s common knowledge, say, that every driver stops for a stop sign – and we shouldn’t take that shared understanding for granted. Steven Pinker is the Johnstone Professor of Psychology at Harvard University. He joins host Krys Boyd to discuss the rules of society we all know and follow and the ways our world would fall apart if people disregarded this social contract. His book is “When Everyone Knows That Everyone Knows . . .: Common Knowledge and the Mysteries of Money, Power, and Everyday Life.”