Judging A Book By Its Cover

If people didn't judge others on superficial appearances, and make assumptions about them based on the way they look, then half the business world would shut down. There would be no booming market for clothing, hair, shiny new cars and trucks, nails, shoes, "beauty" products, make up, plastic surgery, dental cosmetics, or jewelry. There would be no huge market for home improvement or landscaping. There would be no big market for "body art", or shows about "style", and hardly any magazines at all. There would be no sexism, racism, weightism, or heightism.
Denying that it's all over the place just reveals that one does it even more than average, and is trying to hide it.
Just because it's common doesn't mean there's nothing wrong with it. Judging others based on their superficial appearance and then treating them according to these assumptions (which are not based in fact, but in one's own emotions and prejudices) is the single most common form of bullying and discrimination.
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