You may have noticed some differences in political ads this year: They're a lot less negative. But that's one difference that's likely to change.

University of Missouri Communication Professor Bill Benoit analyzed political ads from 1952 to 2004 and found a positive change among this year's ads. This year's Republican ads have been 10 percent more positive than the past 56 years and Democratic ads were nine percent more positive, Benoit said. Benoit breaks down an ad into different pieces to determine its positive or negative ranking, rather than evaluating it on the whole. 

"We found that most ads have a combination of positive and negative statements," Benoit said. "When there are both positive and negative statements in an ad, we can get a more accurate understanding of the content of the ads by looking at each individual statement."

Benoit's analysis suggests that candidate similarities make ads more positive in the primary season, but he expects them to become more negative as the general election approaches.