Must read Farnam analysis in WaPo.  And the evidence bears out the anecdotal reporting that Super PACs go more negative because they are unaccountable: “Data show that super PACs, which have run more advertising than the campaigns themselves, have spent 72 percent of their money on negative ads. The figure for campaigns is 27 percent, according to a Washington Post analysis of data from Kantar Media/Campaign Media Analysis Group, which tracks television advertising across the country. (For this article, ads were considered negative if they mentioned another GOP candidate.)”