Twin peaks: Expressive externality in group participation

We introduce a model of group behavior that combines expressive participation with strategic participation. Building on the idea that expressive voting in elections is much like rooting for a sports team, we give applications to both sporting events and elections. In our model there is an expressive externality: an individual enjoys an event more when more of her peers come out to support her preferred party or team. We show that this results in the possibility of "tipping"-that participation may jump up discontinuously when the externality becomes strong enough. We examine the implications for pricing by sports teams and for voter turnout.