Conformity too. This is a factor often overlooked in discussions of this sort.

(There are in fact two ways in which education signals conformity. The first one is the fact that you have conformed to the social norm that you are supposed to signal your intelligence and conscientiousness with this particular costly and wasteful endeavor, not in some alternative way that would signal these traits just as well. The second one is that you have successfully functioned for several years in an institution that enforces an especially high level of conformity with certain norms of behavior that are especially important in a professional context.)