but he claims that his swearing at home doesn't increase the likelihood of him swearing at school
Tough! Many adults fail to understand that they're not perfect rational agents, so instead that their habits really matter. I guess on the bright side this could be a good opportunity to teach him that he should not encultivate habits that raise the psychic cost of virtuous behaviour, even if those habits are themselves not inherently vices.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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