Miriam believes she is not attractive so she typically wears ugly clothing. as a result, men rarely show interest in her, which confirms to her that she is not attractive. miriam's behavior is an example of a(n

I believe the answer is: self-fulfilling prophecy.

self-fulfilling prophecy refers to the prediction that indirectly cause itself to be true/fulfilled.
We all know that achieving 'beauty' needs a lot of effort such as make up, workout, clothing, etc. In the scenario above, due to her inherent belief of not being pretty, Miriam actually convinced herself that doing these efforts even though she could increase her beauty by doing those efforts.