Negative Advertising and Political Competition

Why is negative advertising such a prominent feature of competition in the US political market? We hypothesize that two-candidate races provide stronger incentives for going negative relative to non-duopoly contests: when the number of competitors is greater than two, airing negative ads creates positive externalities for opponents that are not the object of the attack. To investigate the empirical relevance of the fewness of competitors in explaining the volume of negative advertising, we exploit variation in the number of entrants running for US non-presidential primaries from 2000 through 2008. Duopolies are over twice as likely to air a negative ad when compared to non-duopolies, and the tendency for negative advertising decreases in the number of competitors. The estimates are robust to various specification checks and the inclusion of potential confounding factors at the race, candidate, and advertisement levels.