What research explains political attitudes of academics? According to the Times of Higher Education, there are more liberals* than conservatives in the social sciences. The ratio of Democrats to Republicans in economics is 5:1, in law it is 9:1, in psychology 17:1, in journalism 20:1, and in history a whopping 33:1. 

From my experience, this trend extends outside the social sciences, i.e. to the humanities and the natural sciences, although it may be strongest in the former. I don't have data handy to back this up.

Is there any serious empirical research into why professionals in academia are more likely to vote democratic than the average of the population? If so, what are the main explanations? (Candidates could be self-selection, socialization, or confounding variables like income and education.)


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I really do not think we are going to get to any causal relationship here. One thing you might want to consider is the link between IQ and politics.

Smart people (generally) tend to be more liberal:
https://theconversation.com/do-smart-people-tend-to-be-more-liberal-yes-but-it-doesnt-mean-all-conservatives-are-stupid-57713

Childhood IQ in Britain, for a 1970s cohort, predicts voting for more liberal parties (Greens and Lib Dems):
https://www.research.ed.ac.uk/portal/files/8896159/childhood_intelligence_predicts_voter.pdf

This answer is only meant as a starting point. You would need to consider all of the evidence from across the world (perhaps the reverse is true, in general, for example. And perhaps other variables come into play).