Why do university faculty "lean to the left" ?

Excerpts from an op-ed piece in the StarTribune:

That faculties are liberal is beyond dispute. In a rigorous survey, University of British Columbia sociology Prof. Neil Gross concluded, "professors currently compose the most liberal major occupational group in American society."..

Gross and Solon Simmons of George Mason University surveyed more than 1,400 full-time professors at more than 900 American institutions. Only 19.7 percent of professors identified themselves as "any shade of conservative" (compared with 31.9 percent of the general population), while 62.2 percent identified themselves as some flavor of liberal (compared with 23.3 percent of Americans overall).

Gross found variation between disciplines. Social sciences and humanities contained the highest concentration of liberals. Conservatives were as numerous as liberals in business, health sciences, computer science and engineering.

Postulated reasons for these findings are discussed at the link.