It isn’t just in medicine. It is all of academia and the sciences.
Survey shocker: Liberal profs admit theyd discriminate against conservatives in hiring, advancement
http://www.washingtontimes.com/news/2012/aug/1/liberal-majority-on-campus-yes-were-biased/
The big review paper on the lack of political diversity in social psychology
http://heterodoxacademy.org/2015/09/14/bbs-paper-on-lack-of-political-diversity/
Thanks for those links.