I think it&#x27;s because mobility is easy in the downward direction -- if the banker and the lawyer chose to become the janitor and the homeless guy, they could do that easily.<p>However, the banker and the lawyer realize that they have more life choices available if they are nearer to top of the hierarchy, and this directly leads to more obligations -- to maintain their positions, they must fulfill more responsibilities.<p>More choice leads to more reproductive success.<p>Q.E.D.