In Going Negative: How Political Advertising Divides And Shrinks The Electorate, Ansolabehere and Iyengar demonstrate:

how attack advertisements win elections. Political adverts cost millions and they now increasingly focus on the opponents weaknesses through nasty and personal attacks. Drawing on both laboratory experiments and the real world of America’s presidential, and congressional races, the authors shows that negative advertising drives down voter turnout, and the political consultants intentionally use adverts for this purpose. Among the authors conclusions are that negative adverts work better for Republicans than for Democrats, and better for men than for women. Negative adverts also work better than positive ones, so attacking has become nearly universal. The authors also argue that as independent voters are driven away by all this negativity, the voting public is increasingly reduced to partisan extremes.