A new study finds even the most well-behaved police officers tend to speak less respectfully to black residents than to white ones.

An analysis of transcripts from 981 traffic stops revealed racial disparities in officers' language that were subtle, but widespread, according to the report published June 5 in the Proceedings of the National Academy of Sciences. And the findings hold true regardless of a police officer's racial background, the crime rate of the area or type of neighborhood of the stop.

A team led by Stanford University graduate student Rob Voigt transcribed a month's worth of recordings of traffic stops conducted by the Oakland Police Department. Nearly 250 different officers conducted the traffic stops, which took place in April 2014 and involved 682 black drivers and 299 white motorists.

The study consisted of three phases. First, participants rated a portion of the transcribed conversations based on how respectful, polite, friendly, formal and impartial the officers' language was. Participants weren't told the race or gender of those involved in the conversation. Researchers then used the fine-tuned rating model on the entire sample and highlighted language shifts.

Researchers reviewed roughly 36,700 comments, or utterances, from officers and found white residents were 57 percent more likely than black residents to hear a police officer say the most respectful comments. Respectful interactions included ones in which officers apologized and expressed gratitude, saying things like "thank you."

Conversely, black motorists were 61 percent more likely than whites to hear an officer say the least respectful things. In those cases, officers used informal titles like "dude" and "bro" and made different commands like "hands on the wheel."

At no point was there swearing.

"These were well-behaved officers. But the many small differences in how they spoke with community members added up to pervasive racial disparities," said Dan Jurafsky, a co-author in the study, said in a statement.

Researchers warned that shift in respectful language could erode police-community relations.

"Understanding and improving the interactions between the police and the communities they serve is incredibly important, but the interactions can be difficult to study," Jurafsky said. "Computational linguistics offers a way to aggregate across many speakers and many interactions to detect the way that everyday language can reflect our attitudes, thoughts and emotions – which are sometimes outside of our own awareness."

The study stressed that the findings "are not proof of bias or wrongdoing on the part of individual officers." Different facts could drive the racial disparities, researchers noted.