According to professor William Schenck-Hamlin, "The use of negative political ads has increased dramatically over the last 25 years."
Many people have noticed that this election has an awful lot of negative advertising.
Allow me to submit a reason: the candidates suck.
In social situations, if you haven't anything nice to say then the rule is to say nothing at all. However, in politics, if there's nothing nice to say about yourself, then you need to say something bad about your opponent.
Essentially, each candidate is saying that if even if you think of him as bad, the other guy is worse, and bad is preferable to worse.
Hmm. Any wonder why we end up with so many bad presidents?