I find myself very vague most of the time, especially with people who are a lot older than me (even though I get along better with older people). It's just that older people speak to me like I am vague, so I feel like I'm vague. (Youngsters recognise my weirdness but not vagueness, and older people recognise my vagueness but not so much my weirdness).
But I don't know whether vagueness can be a personality trait, or if it's due to lack of confidence. I don't think it's part of my personality, because I don't act so vague when I'm with my family. Sometimes I can be vague when it comes to maths, politics or science, but I'm talking more about social vagueness here. It seems it is lack of confidence for me, because I seem to come across as more vague when I'm speaking to people who I feel quite nervous of shy with. So it can depend on what type of people I speak to.
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Female