Joshua Correll of the University of Colorado at Boulder has used an online shooter video game to try to measure these unconscious attitudes (you can play the game yourself). The player takes on the role of a police officer who is confronted with a series of images of white or black men variously holding guns or innocent objects such as wallets or cellphones. The aim is to shoot anyone with a gun while holstering your weapon in other cases.
Ordinary players (often university undergraduates) routinely shoot more quickly at black men than at white men, and are more likely to mistakenly shoot an unarmed black man than an unarmed white man.
I’m typical. The first time I took the test, years ago, I shot armed blacks in an average of 0.679 seconds while waiting slightly longer — 0.694 seconds — to shoot armed whites. I also holstered more quickly when confronted with unarmed whites than with unarmed blacks.
In effect, we have a more impulsive trigger finger when confronted by black men and are more cautious with whites.