Political attack ads are being analyzed by a communication professor at the University of Missouri-Columbia, who says this year’s presidential candidates are generally running positive ads. But Professor Bill Benoit says the political parties’ ads on behalf of the candidates are more likely to be negative. Benoit says attack ads go back to the beginning of presidential advertising on TV. He says close races generate relatively low percentages of negative ads. But he says candidates are more likely to become heavily negative as they call behind.