This is less a discrete fact and more a body of experience that influenced my view here, but I think it still falls under the topic. Part of me feels like this should be obvious to the average LWer, but on the other hand it seems to be a very common malfunction among adults.

When I was a kid, probably until sometime in High School, I thought that if I saw someone wearing, e.g.

• a backwards hat, an overlarge basketball jersey, and sagging jeans, or
• Spanish-language tattoos, thick gold necklace and stud ear piercings, or
• a black leather jacket, spiked dyed black hair, facial piercings, or
• ... some other stereotype which is barred from American public school dress code ...

... I would believe that this individual was probably actually dangerous. I would be frightened by the sight of them, and believe that there was a good chance that they would mug me.

Eventually I realized that these are merely the uniforms of various subcultures loosely enveloped by ethnic lines, and that seeing somebody wearing "thuggish" attire is very very weak Bayesian evidence that they are actually a thug, especially if you encounter them in a shopping mall or walking their dog at a park.

The most interesting part of realizing this was seeing that many adults don't seem to ever realize this, and consequently end up repeatedly outing themselves as casual racists. Has anyone else ever heard that a certain area of their city was "the bad part of town," and that they should never go there, and then later realize (after perusing crime statistics and visiting that area) that the only "bad" thing about the area is that it's predominantly Hispanic (for example)? A surprising number of people I know, primarily those raised in the suburbs, go through life burdened by an unfortunate degree of unnecessary fear.

You can also have fun with this knowledge about human perception. It is trivial to transform yourself into someone that other people would not want to mess with with a few superficial choices in clothing.