Their bubble of awareness is smaller.
The smaller the bubble, the less consideration of others come into play, because they are less aware of others. When it is pointed out that others are there, they say, "Oh."
It's why kids can be loud at church, or people can eat boogers in public...even though others can see and hear them, and they should be embarrassed, dumb people or young people or drunk people are unaware of others around them. Their bubble of awareness barely encompasses themselves, and it definitely excludes most others.
Civility is also a factor, of course. But one indicator of increased intelligence is a larger bubble of awareness.
Just my theory.
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