Ten-thousands years ago, “I scratch your back, you scratch mine” was a survival tactic reserved for families. But soon our species learned friendly vis-à-vis with strangers offered even greater chances of success. Michael McCullough, professor of psychology at the University of California San Diego, joins host Krys Boyd to talk about how our collective experiences have taught us to care for one another. His book is called “The Kindness of Strangers: How a Selfish Ape Invented a New Moral Code.”