Some campaign ads from the 2008 presidential election between Sens. John McCain and Barack Obama may have been designed to rouse racial biases in some voters, a study says. Published this month in Public Opinion Quarterly, the study finds that attack ads depicting Obama with darker skin may have been intended to influence views of the Democratic senator from Illinois, who went on to win the presidency. 

Researchers looked at 126 ads that aired during the 2008 campaign and digitally evaluated the two nominees' skin tone in each spot. The ads were then categorized based on themes. 

In negative advertisements, both campaigns often would either shadow or wash out the other nominee's face. Researchers found that when McCain's campaign would run a commercial that linked Obama to liberal groups' alleged criminal activity, images of Obama would usually show him with darker skin.

It is unclear if this was intended by the McCain campaign, but as the date of the election neared, Obama gradually was depicted as darker in the Arizona senator's advertisements, while McCain appeared to be somewhat more washed out. 

In total, the study found that 89 percent of McCain's ad featured Obama where "his skin tone was in the darkest quartile of all ads studied," the Chicago Tribune reported.

Researchers found these images of Obama had an effect on viewers. After viewing the ads, respondents were asked to fill out a crossword puzzle-type game. They were offered the letters "L A" and required to fill in two blank spaces that followed.

Those who viewed images of Obama with lighter skin filled in the word "lazy" 33 percent of the time, while those who saw the darker image wrote the word "lazy" 45 percent of the time. Researchers said that the findings reflect viewers "having more negative stereotypes on their minds after seeing the photography," the Chicago Tribune reported.

The study was conducted by Solomon Messing of the Pew Research Center; Julia Mabon, a software engineer who works for LinkedIn; and Ethan Plaut, a postdoctoral fellow at Stanford University.