I think you have taken a couple of assumptions and drawn the wrong conclusions. First, introversion/extroversion describes a characteristic of how you generall

I think you have taken a couple of assumptions and drawn the wrong conclusions. First, introversion/extroversion describes a characteristic of how you generally prefer to be 'energised' (introvert: on your own; extrovert: around other people). Whilst the description is a binary one, the characterisation of someone is not. If you ever do a Myers-Briggs test, the result you get back will often place you on a sliding scale of introversion-extroversion - when I did the test I was on the border of the two (slight leaning towards introversion). So simply because your ex decided to have a quiet night in, doesn't suddenly means she's an introvert. Unless one is an extreme introvert or extrovert, you won't be solely displaying the one type of behaviour at all times. Second, the 'state of mind' conclusion seems to be a bit of a leap of logic :) Just because someone exhibits the occasional different behaviour doesn't mean it's 'all in the head'. Would you argue that cancer suffer who have good and bad days with their pain mean that their pain is caused by a 'state of mind' rather than an actual illness? Introversion/extroversion is not something you can 'think positive' your way out. This is exactly the complaint of introverts who are tired of people who think that 'if they just made more effort they wouldn't be so anti-social'!