The sociologist Andreas Diekmann analyzed the bystander effect as a game he called the Volunteer’s Dilemma. In the Volunteer’s Dilemma, the strategy is to volunteer with a probability that depends on the relative costs of no one helping, someone helping, and oneself helping—and on the number of potential volunteers. Notice that the entire scenario presumes common knowledge, which is “knowledge of others’ knowledge, knowledge of their knowledge of one’s knowledge, ad infinitum.” Common knowledge is the subject of my latest book, When Everyone Knows That Everyone Knows…. With private knowledge, unlike common knowledge, responsibility needn’t diffuse as the number of potential volunteers increases. And they behaved, more or less, like recursive mentalizers in a Volunteer’s Dilemma should, zigzagging in their volunteerism with the level of embedded knowledge.