так вот, наболело. особенно после прочтения (действительно, кстати, хорошего) рассказа (http://roberlee.livejournal.com/289508.html)
(далее обсуждение в терминах рассказа - без прочтения непонятно)

it is a known, and much advertised fact that in US a policeman is much more likely to shoot (by mistake) a black person than a white one.
there were cases when policemen shot black people that were holding objects such as wallets, keyrings, candy bars etc because they (policemen) thought that they (the black people subsequently shot) were holding a gun. 

now, cultural biases, criminality ratios and racial tensions nonwithstanding - may it be possible that it is just harder to distinguish a black gun in black hand from any other black object in black hand, than a black gun in a white hand from any other black object in a white hand? especially under poor lighting, sub-second reaction times etc.? could simple lack of contrast explain some of that (highly politically charged) fact?

easy to test - just make a lot of pictures of common objects in differenet colours held in hands of different colours, show them to people and ask whether it is a gun or not.

I wonder if anyone did it already?