A phenomenon wherein people's attitudes, beliefs or assumptions about another person (or persons) can, with or without their intent, actually produce the very behaviors that they had initially expected to find is called:

Self-fulfilling prophecy

Explanation:

Self-Fulfilling Prophecy is that phenomenon wherein people's attitudes, beliefs, or assumptions about another person (or persons) can, with or without their intent, actually produce the very behaviors that they had initially expected to find.

This means that the exact behavioural pattern you expected of or from an individual turns out the exact way you anticipated.