When Bernard Demczuk does diversity and cultural awareness training with police, he gives a history lesson. Call it: why black people distrust police.

He uses a timeline, starting with the role of slave catchers in the United States in 1706 to police black bodies to the enforcement of Black Codes and Jim Crow laws passed during the 19th and 20th centuries to restrict African-American freedoms.

He notes the role of law enforcement post-emancipation in supporting the system of convict leasing, which provided prisoner labour mostly made up of black men.

Finally, he connects those early efforts in policing, through official and unofficial channels, to today’s “stop and frisk” policies and the disproportionate use of deadly force towards African-Americans.

“What I do in my class, I say to police, ‘Guys and gals, you have been on the wrong side of history for 310 years,’ “ Demczuk said. “You may be trying to do the right thing, but if you don’t know the history, you might not know what the right thing is.”

For the past year, Demczuk, who holds a doctorate in African-American history from George Washington University, has been teaching these lessons at the Metropolitan Police Academy in Washington.

More than 2,000 officers have been through the programme and another 2,100 are scheduled to take the class.

In February, he said, officers from Anne Arundel County, Hyattsville and Cheverly will be visiting his class to learn more about his training model.

He recently offered his services to the town of Greensboro, which continues to roil from a police-involved death last September of an unarmed black resident.

The town has a four-man police force, including the chief. But when it comes to friction between black people and law enforcement, the size of the police department doesn’t matter.

In September, police received a call about a man “dragging an unidentified 12-year-old down a street”.

When an officer responded, Anton Black, 19, said he and the young boy were brothers. They were not.

Both ran from the officer. He pursued and was joined by two off-duty officers from a neighbouring town and a civilian on a motorcycle.

Video taken from the Greensboro police officer’s body camera was released last week.

Black was “schizophrenic”, one of the officers radioed in during the chase.

They found Black hiding in a parked car, and a struggle ensued. Black was wrestled to the ground on his stomach. One person held him by the head, another held his feet, a third lay across his back while a fourth put his knee on Black’s shoulder.

A few minutes later, Black’s body went limp. He was taken to a hospital and pronounced dead.

“Nothing we could do,” one of the officers was heard saying on the bodycam video. “He had superhuman strength. It scared me.”

That reasoning is part of a familiar pattern, Demczuk said. It was used to justify the beating of Rodney King by four officers in Los Angeles in 1992.

It was used by New York police officers in the chokehold death of Eric Garner in 2014.

It was used to explain the shooting death of Michael Brown by a police officer in Ferguson, Missouri, also in 2014, Demczuk said.

“We have been predisposed to see black men as predators for over 400 years,” Demczuk tells the officers in his classes. “Part of white supremacist ideology is that the black man could be killed with impunity because white women needed to be protected from them.”

Demczuk resides in Caroline County, Maryland, where Greensboro is located. So, his offer to help the Police Department is both professional and personal.

“I want to help my local police departments,” he said. “We can learn to confront implicit bias and find new ways to confront suspects that de-escalates the chance of violence rather than make the situation worse.”

The Greensboro Town Council is expected to vote soon on whether to bring in a diversity trainer. Some, including a few of Black’s relatives, still don’t quite understand what good that will do.

“Why do you need to teach four grown men how to act like decent human beings?” said LaToya Holley, Black’s sister.

The way Demczuk sees it, even decent men will embrace police culture without realising how much racial baggage they’ve taken on.

“One of the questions that police ask about the Anton case is, why did he run?” Demczuk said.

Set aside that the first officer to confront Anton had been videotaped kicking a black suspect in the face at his previous job.

In his classes, Demczuk looks at the cumulative effects of racist policing.

Police officers enforced laws backing segregation, often with excessive force and violence.

Historically, they did little to stop or investigate the thousands of lynchings throughout the United States.

“And you ask, ‘Why did he run?’,” Demczuk asked.

The purpose is not to guilt-trip the police, he notes, but simply to let them know where the mistrust comes from and the police behaviours that perpetuate it.

“We can’t change the past,” Demczuk said. “But if we understand it, we can change the future.”

•Courtland Milloy is a local columnist for The Washington Post, where he has worked since 1975. He has covered crime and politics in the District of Columbia and demographic changes in Prince George’s County, Maryland. He has also written for the Post’s Style and Foreign sections