Part of the reason [conservatives are far less likely to express interest in pursuing a PhD] stems from their preferred majors. Conservative students are approximately twice as likely to major in professional fields such as accounting or hotel management that focus on immediate employment rather than advanced study.Beyond just majors or disciplines, I think this indicates something more radical. Yes, part of this is a selection effect, but there is at least in part some actual reflexive causality here. Education per se and the liberal arts in particular are progessivizing. We demand that students critically engage material and understand that material in context. By definition, this epistemological process pushes thinkers to the left. Those who embrace liberal arts majors (e.g. sociology) are going to get more of this than those who embrace professional programs (e.g. business). Anyone, however, who is educated in a traditional way is likely to be pushed to the left--not by agenda or bias but by the indirect influence of the very process of education.
: Just so there is no confusion, the "liberal" in liberal arts is not the same "liberal" as in a left-leaning political philosophy. The liberal arts was initially a course of education befitting a free man. (The Latin word for free was liber.) This was different from education befitting slaves and the working class.