Popular television shows that put black and white characters on an equal footing as doctors or detectives may still be transmitting racial bias nonverbally, according to a new study. Researchers found that in a selection of brief, silent clips from 11 prime-time television shows, white characters were consistently rated as behaving more positively toward other white characters than toward black ones, even when the black characters were deemed equal to their white counterparts in attractiveness, kindness and intelligence.

The study, published on Thursday in the journal Science, suggests one way racial biases could be propagated even when people are consciously on the lookout against racist stereotypes or remarks. "What this suggests is the media is one of the mechanisms of how we see people's true feelings and therefore how we form our own biases," says John Dovidio, a social psychologist at Yale University who did not take part in the study. "The nonverbal behavior really conveys what people are thinking and feeling, and that's why it's so potent."

Prior studies have found that even if people don't act racist in obvious wyas, they still tend to make subtle racial judgments, such as staring longer at black customers browsing in a store than at white ones. To see whether nonverbal behavior might transmit such biases, researchers from Tufts University selected 10-second clips from shows such as Scrubs , House and Grey's Anatomy , which feature black and white doctors in starring roles, and other programs including CSI and Friday Night Lights.

To assess whether characters of both races were being treated equally, the researchers cropped out a single character from each clip, either white or black, and turned off the sound. Then they asked white college students to rate how much the remaining on-screen characters, who were white, liked or were positive toward the cropped-out character. After averaging the students' ratings, the researchers found that in nine out of 11 shows, white characters exhibited less favorable reactions toward cropped-out black characters than toward white ones. In the two shows where black characters were more favored, the bias toward them was less pronounced than the bias toward whites in the other shows.

The researchers then reinserted the cropped-out characters and prepared selections of silent clips in which the favorability ratings of the featured white characters were higher than those of the featured black characters. After watching the videos, students were quicker to associate white faces than black ones with positive terms such as peace and love. (Scientists say these so–called implicit associations capture the potential for subtly biased behavior.)

When researchers reversed the pattern and showed students videos in which the black characters were the ones treated more favorably, the students were then quicker to associate positive words with black faces. Students exposed to the pro-black videos also expressed less prejudice against black people on a standardized questionnaire.

The effect seemed to be based on body language and facial expressions that the viewers were otherwise unable to recognize consciously. The researchers randomly assigned students from a separate group to watch either the white- or black-biased videos and instructed them to look for a hidden pattern. After the fact, the students were unable to correctly guess whether the set of videos they had seen favored whites or blacks.

The researchers say the results don't point to racism in particular shows but to a pattern across many shows that viewers may pick up on without realizing it, leading them to associate positive qualities with white characters. "We do believe that people who don't start out as explicitly biased could be nudged in that direction by watching enough popular TV shows that exhibit nonverbal race bias," says social psychologist Max Weisbuch, the study's lead author. In a separate group of students, the more of the 11 shows they reported watching, the quicker they were to associate positive terms with white faces, even without viewing the silent video clips.

The findings do point to the power of facial expressions in transmitting bias, but that doesn't mean they reflect people's actual viewing habits or their responses to the full shows in their real context, says Diana Mutz, a professor of communications and political science at the University of Pennsylvania. "It's looking at a very subtle kind of influence that may be swamped by more obvious things in the programming," she says.