And All Painters Wear Berets...

I do think there is something to artists generally being "not-so-good" at math, but maybe that's because I don't have a mathamatical bone in my body and I was born into an "artsy" family. Stereotypes are generalizations and even if many people do fall into a category, there are plenty of others who do not. My understanding is that stereotypes are based on the people that draw attention--the extreme. For example, the extremely liberal writers tend to voice their beliefs publicly (duh--they are writers) and therefore that is what the public references. "Oh, I read this liberal article in the New York Times the other day so obviously that is a liberal newspaper." Well, that (hypothetical) person may have read an article he/she believed to be liberal but that may also be the only article he/she read over the course of a week. It all needs to be contextualized.
Another example of this phenomenon is the stereotype in America today that all Christians are Evangelical/bible-banging/conservatives. Why? Because the extreme Christians who eat, sleep and breathe and above all SPEAK conservative principles are usually the Christians that draw the public eye (I am sorry to say).
The same is true of Journalists being "heartless." Charlie Gibson was criticized for "being too hard" on Sarah Palin during the exclusive interviews, but people forget that journalists are SUPPOSED to be the "watchdogs of the government" he was just doing his job. And as a side comment--if Sarah Palin can't hold her own in an interview with Charlie Gibson than she has no business in the White House--I digress...
The point is, human nature latches on to perceived common denomenators (Wow I actually used a math term) or similarities amongst groups of people. Stereotypes come out of this avenue of human nature. It doesn't mean that ALL people are _____, it means that the public perceives a group of people to be a certain way. Perception is different than truth.