Discrimination against people on the basis of their observable group traits is an enemy of equality of opportunity. This is a common-sense observation. If all discrimination were statistical discrimination, the common-sense observation would be true only to a very limited degree. If observable group traits such as skin color or sex are correlated with traits that employers reasonably value, hiring on the basis of the observable group traits can be rational from the standpoint of the employer seeking both to hire qualified workers and minimize hiring search costs.