You can't police people into not being racist. People have always been racist/xenophobic to some extent and always will be. It's cultural conflict and tribal in nature.
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Arguing from human nature isn't compelling. Rape and murder are part of human nature as well, and people have always done both, yet it isn't controversial to police such behaviors. Racism is no different. We aren't mere animals entirely beholden to our base instincts, after all.