At the turn of this century, most banks required tellers to have a high school diploma. Even though the tasks currently performed by tellers in most banks can still be performed by persons who have mastered the high school curriculum, many banks now require that their tellers have college diplomas. Assuming that the real costs of college education are lower now than at the turn of the century and that these costs are lower for persons of higher ability, construct an explanation for why banks might have raised their hiring standards.