The historian KC Johnson has an interesting, link-filled article on ideological and intellectual diversity at Inside Higher Ed:

Inside Higher Ed recently reported on four University of Pittsburgh professors critiquing the latest survey suggesting ideological one-sidedness in the academy. According to the Pitt quartet, self-selection accounts for findings that the faculty of elite disproportionately tilts to the Left. "Many conservatives," the Pitt professors mused, "may deliberately choose not to seek employment at top-tier research universities because they object, on philosophical grounds, to one of the fundamental tenets undergirding such institutions: the scientific method."

Imagine the appropriate outrage that would have occurred had the above critique referred to feminists, minorities, or Socialists. Yet the Pitt quartet's line of reasoning — that faculty ideological imbalance reflects the academy functioning as it should — has appeared with regularity, and has been, unintentionally, most revealing. Indeed, the very defense offered by the academic Establishment, rather than the statistical surveys themselves, has gone a long way toward proving the case of critics who say that the academy lacks sufficient intellectual diversity.

Johnson then critiques three excuses for ideological homogeneity:

1. The cultural left is, simply, more intelligent than anyone else. As SUNY-Albany's Ron McClamrock reasoned, "Lefties are overrepresented in academia because on average, we're just f-ing smarter." The first recent survey came in early 2004, when the Duke Conservative Union disclosed that Duke's humanities departments contained 142 registered Democrats and 8 registered Republicans. Philosophy Department chairman Robert Brandon considered the results unsurprising: "If, as John Stuart Mill said, stupid people are generally conservative, then there are lots of conservatives we will never hire." . . .

2. A left-leaning tilt in the faculty is a pedagogical necessity, because professors must expose gender, racial, and class bias while promoting peace, "diversity" and "cultural competence." According to Montclair State's Grover Furr, "colleges and universities do not need a single additional 'conservative' .... What they do need, and would much benefit from, is more Marxists, radicals, leftists — all terms conventionally applied to those who fight against exploitation, racism, sexism, and capitalism. We can never have too many of these, just as we can never have too few 'conservatives.'" . . .

3. A left-leaning professoriate is a structural necessity, because the liberal arts faculty must balance business school faculty and/or the general conservative political culture.