Incidentally, this also explains why median US voter is to the right of median EU voters (US was founded by risk-takers, people who left their old lives behind), and the difference between men and women (men are risk-takers).

Two other comments:

To me, “professional” has always seemed the closest thing to “academic” without being actually in the academy, (or semi-academy of think tanks). Has the distribution of academic voting changed over the same period?

Also, I would have thought that an ever increasing percentage of “professionals” are professional in guiding people through the arcane intricacies of regulations. I would expect those folks to favor pro-regulation politicians.

Here’s another thought. Maybe I missed it, but I didn’t see any indication that gender was controlled in these findings? While women have always outnumbered men in teaching and nursing, more traditionally male-dominated professions have seen a tremendous rise in the percentage of women in the ranks over the past twenty years. I wonder if this might account for some, or all, of the shift toward a preference for Democrats among professionals.

Damn you government! A weaker minded liberal might have lost his tax and spend ways.

People hate talked about some of these things, for example the idea that many professionals feel more like employees than free agents. For example, if you’re a doctor you might be more likely to be annoyed at insurance companies than at the government. But I don’t know any systematic research on this. Also, not all the professionals, owners, etc., are wealthy.

Regarding your questions about personality variables predicting liberalism or conservatism, check out the work of John Jost. I’m not sure if they’ve connected these to occupation.