There are two very different accounts of what prompted a Ferguson, Missouri police officer to shoot and kill Michael Brown on Saturday. The unnamed police officer who shot the 18-year-old claims he was “physically assaulted.” Witnesses said Brown had his hands in the air to show he was unarmed, and the officer fired from 35 feet away.

Before Brown, there was 16-year-old Kimani Gray, 19-year-old Kendrec McDade, and countless other unarmed African Americans killed by police gunfire. We don’t need to know the outcome of the FBI’s investigation and the Department of Justice’s separate “fulsome review” into civil rights violations to recognize the racism in our justice system.

But racial bias also factors into officers’ split-second decision to shoot a suspect.

Social science research shows that, in video simulations, people are more likely to shoot black men. The participants—often undergraduate students, both black and white—play a simulation where they press “shoot” if they think the white or black suspect holds a gun. Consistently, psychologists have found the students more likely to shoot the unarmed black person over an unarmed white person.

For example, a study published in 2002 from the University of Colorado at Boulder and University of Chicago found that white undergraduates had higher error rates when it came to unarmed African American suspects (1.45 per 20 trials compared to 1.23 for unarmed white suspects).