I haven't conducted a study, and I'm not a sociologist or a psychologist, so I can only answer the question based on my own personal experience and ... er ...research. Lucky for all of you, that research has been pretty extensive (let's keep that between us, K?)
Substance Abuse Counselors - See above. They seem to be convinced they know everything about interpreting your actions and words. They also seem to be pretty good at finding a way to blame the whole thing on you, anyway. Personally, I think this is because they are often former addicts, and that's what addicts do. Or maybe it's because they spend so much of their time counseling others in how to own their problems, and they forget they aren't counseling their date to do the same. Either way, in order to date this guy, you need to be ready to take on a whole lot of blame.
Law Enforcement - This includes police (state, county, municipal), other law enforcement agencies, and corrections officers. These guys are historically bad at commitment, and are known for being "dogs." Personally, I think it's because they have to separate themselves from their work, and maybe that disassociation spills over into their personal life. Or maybe it's just because one needs a specific mind-set to do any of these jobs, and that personality trait makes for a bad date. Either way, if you're dating this guy, be prepared that he may not be in it for the long run. Also - his hours will probably suck.
Car Salesman - These guys come off shady, maybe because they make their living telling people what they want to hear in order to close a deal. That definitely spills over into their personal life. This guy will be able to read you like a book (better than any counselor ever could) and will be saying all the right things within minutes. If you're going to date this guy, be prepared that most of what he says is simply not true.
DJ, Musician, Artist, or other creative types - First of all, let's be practical - unless you're dating Josh Groban or Jay Z or James Patterson, this dude has no money. You will get stuck paying for a lot, and probably driving everywhere. It's one thing to always pay your own way - but to have to pay his, too? Just something to consider. Like law enforcement, these hours also suck. Even worse than the hours is the fact that creative types have no concept of time. They get wrapped up in their project, with no regard for you or your plans or your feelings. If you want to date this guy, be prepared to never be in his starting lineup.
Self-Employed - There are definite and obvious exceptions to this observation, but basically...this guy is two steps away from unemployed. Just keep that in mind, because really, if you wanted someone to take care of, you could go to the shelter and adopt a pet.