Young black men are viewed as larger and more threatening than their white counterparts, according to a new psychological study.

Nearly 1,000 people from the US were shown pictures of the faces of white and black men and asked to guess how big they were and whether they might be dangerous.

The researchers said the results showed black men were seen as “larger, stronger and more muscular” than white men even when they were the same size.

This was true of both black and white participants in the study.

However only white test subjects judged young black men to be more of a threat and that police might be more justified in the use of force to subdue then, even when they were not armed.

A series of killings of unarmed black men – by police and self-appointed vigilantes like George Zimmerman, who killed teenager Trayvon Martin – have sparked the Black Lives Matter campaign in the US.

The researchers suggested that part of the problem could be the perception that black men were more of a threat.

In a paper in the Journal of Personality and Social Psychology, they pointed to the 2014 killing of an unarmed black man, Dontre Hamilton, by a white police officer in Milwaukee.

The officer claimed Mr Hamilton, who was shot 14 times, had a “muscular build” and “most definitely would have overpowered . . . me or pretty much any officer I can think of, to tell you the truth. He was just that big, that muscular”. Mr Hamilton was 5ft 7in and about 12 stone (169 pounds).

Researcher Dr John Paul Wilson, of Montclair State University, said: “Unarmed black men are disproportionately more likely to be shot and killed by police, and often these killings are accompanied by explanations that cite the physical size of the person shot.

“Our research suggests that these descriptions may reflect stereotypes of black males that do not seem to comport with reality.”

The study had found that estimates of people’s height and weight were “consistently biased”, he said.

“Participants judged the black men to be larger, stronger and more muscular than the white men, even though they were actually the same size,” Dr Wilson said.

“Participants also believed that the black men were more capable of causing harm in a hypothetical altercation and, troublingly, that police would be more justified in using force to subdue them, even if the men were unarmed.”

However he cautioned that the study had not simulated a real-world situation.

People’s perceptions – and their memory – can be altered when they are in stressful situations.

Other researchers have suggested that racism stems from an “implicit bias” in the brain against anyone who looks different to the cultural norm.

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Speaking at the annual meeting of the American Association for the Advancement of Science (AAAS), Professor Lydia Villa-Komaroff said the brain had developed a number of shortcuts which could produce a “cognitive illusion” of knowledge, which she described as “a little bit like an optical illusion”.

“So it’s not just a white man or woman who will associate a large black man with possible danger, a black man and a black woman will have that same reaction – in the American culture,” she said.