We present results from a new paradigm: mass participation games. In our experiments, hundreds of people can play a computer game simultaneously using audience response handsets. We can collect responses from a lecture hall full of people with the precision of a laboratory cubicle. We have studied two games: continuous, action games where participants cooperate to achieve a goal; and decision- making paradigms in which participants make repeated choices to maximise their own or the group’s rewards. We address a range of theoretical questions with experimental manipulations and computer modelling. Do participants play as if they were alone, or as a group? If so, do they represent the group as a single entity, or a collection of other agents? What are the dynamics of these behaviours, with learning across many trials? Lastly, what does it feel like to act in concert, or in competition, with a room full of people?