In his new book, Steven Pinker asks us to look at how group knowledge works. Pinker argues that what drives society is knowing that what we know is widely know — in his term, "common knowledge". But what if our beliefs about everyone else's inner thoughts turn out to be wrong?

• Guest: Steven Pinker, experimental psychologist and the author of many popular books on the human mind. His latest is When Everyone Knows that Everyone Knows...: Common Knowledge and the Science of Harmony, Hypocrisy and Outrage
• Producer: Catherine Zengerer and Alex Tighe