If men define their situations as real

Respond to the following prompt no less than 300-400 words:
The Thomas Theorem states, “If men define their situations as real, they are real in their consequences.” This is the “social constructionist” view on society: we create realities in the social world simply because we believe something is real or necessary, and act accordingly. What does this mean to you? What is a real-life example of this theorem, and how does the theorem apply to that example?