Common knowledge is necessary for making arbitrary but complementary choices like driving on the right, using paper currency, and coalescing behind a political leader or movement. Steven Pinker shows how the hidden logic of common knowledge can make sense of financial bubbles and crashes, revolutions that come out of nowhere, the posturing and pretense of diplomacy, and the eruption of cancel culture. Consistently riveting, When Everyone Knows That Everyone Knows invites us to understand the ways we try to get into each other's heads.