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.