We begin a series on evolved psychological mechanisms for group participation, evaluating the wild variation in behavior and belief that groups exhibit. We'll start with dancing epidemics, divine madness, possession states, culture bound syndromes, and a host of other particularly idiosyncratic forms of social contagion. We'll discuss the (unnecessary) tension between 'cultural' and 'psychological' explanations in academia, and how a more useful frame is simply the ability of groups to cohere around beliefs and behaviors utterly unfamiliar to other groups.