(CNN) Police officers conduct themselves differently during traffic stops with Black and White drivers, even down to the subtlest of details, new research suggests.
A study from the American Psychological Association finds that police officers exhibit less warmth and respect in their voices when talking to Black drivers than they do with White drivers.
Researchers sought to examine how officers sound during interactions with Black and White Americans and how their tone of voice affects institutional trust of law enforcement.
"One of the things that was missing from [previous] studies was that it matters not just what people say, but how they say it," said Nicholas Camp, an assistant professor of organizational studies at the University of Michigan and lead author of the study.
"One goal of this research was to see whether we would pick up on racial disparities in more subtle aspects of communication."
Using body camera footage from a month of routine traffic stops in an unnamed, mid-sized US city, researchers had more than 400 participants listen to audio from driver-police interactions and rate how tense, friendly and respectful officers' tone of voice sounded.
The data was collected before the murder of George Floyd in May 2020 and the subsequent national uprising over police brutality.
The audio was edited so that listeners could only hear the police side of the interaction and couldn't discern what was being said -- the clips they heard were akin to how the adults in "Peanuts" cartoons sound, Camp said. Because a driver's language and behavior can potentially influence how an officer communicates with them, researchers specifically used audio from the earlier part of stops when officers explained why people were being pulled over. They also accounted for whether the driver received a ticket or had their vehicle searched, Camp added.
What the authors found was that there were disparities in how police officers responded during interactions with Black and White men.
The study's participants were more likely to perceive officers as talking down to Black drivers and less likely to rate them as friendly during those interactions -- that was the case regardless of the participant's race, gender or ethnicity.
"What this research shows is that these interactions differ not just in what happens in the encounter but also in these interpersonal aspects, like how officers communicate," Camp said. "This matters."
The APA has advocated for community-based policing and initiatives intended to reduce racial bias in law enforcement.
Racial disparities in how officers sound when talking to Black and White drivers further erodes the trust communities have in law enforcement, Camp said.
That's especially concerning after Americans' confidence in police hit an all-time low last year, with Black respondents' trust sinking even further.
"One of the most important tools that officers have at their disposal with the public is their communication," Camp said. "Communication and this interpersonal aspect of policing is undervalued and sometimes overlooked."
CNN law enforcement analyst and former Washington, DC police chief Charles Ramsey said that while "tone of voice does matter," the tone that police take during traffic stops depends on a number of circumstances, such as whether the person is being pulled over for a minor traffic violation, driving while intoxicated or a felony stop.
"All of those are different factors that account for different tone of voice, different commands and things of that nature," Ramsey said. "But officers should always be respectful. They should never be disrespectful."
The APA study, however, focused on routine traffic stops, meaning interactions during which no arrests were made.
Researchers also found a cycle of distrust stemming from disparities in police treatment.
Participants who previously felt they were treated unfairly by police heard less warmth, ease and respect in officers' tone of voice. In another experiment, people who heard negative-sounding audio from police were more likely to think that officers in those departments would be accused of racial profiling or have a complaint filed against them.
"We know from previous research that people base their trust in law enforcement based on their personal experiences," Camp added. "We show that these are institutional interactions -- that things like an officer's language or tone of voice, just very human parts of their communication, matter for community members' trust in the police."
Previous research has shown that law enforcement are more likely to pull over Black motorists than their White counterparts, and less likely to use respectful language in their interactions with them.
Meanwhile, traffic stops have turned fatal in a number of high-profile cases, including the killings of Daunte Wright in April and Philando Castile in 2016. A 2015 analysis by the Washington Post found that a disproportionate share of those killed in such stops are Black.
While Camp's study looks at a narrow aspect of traffic stops, he said he's interested in examining other factors at play during these interactions.
Future research might focus on what aspects cause a police encounter to go awry, what interventions could help de-escalate it and how police departments can ultimately build trust with communities, he added.