Earlier in my career I spent a number of years doing comprehensive psychological evaluations of applicants to local police forces. Your statement is false, and I challenge whether any such "study" ever existed. Lots of psychologists are liberal morons, to be sure, and may hold that private opinion, but there is no study characterizing police officers this way.
Are there bad cops? Of course. We sometimes rendered a "do not hire" recommendation to our departments. They universally followed this guidance in the small number of cases where it was indicated. So what happened to these psychological rejects? They got jobs with departments that didn't bother using serious psychological evaluations.
You can read the hilarious article here:
http://www.examiner.com/article/police-officer-trying-to-shoot-dog-shoots-another-officer-instead
ROFLMFWSCAO!!!!!!
8^)