In a way, I tend to agree with the Avenue Q song "Everyone's a Little bit Racist." Even if we don't believe that any one race is superior to another, we all go through life with our views of others shaped by our encounters with people or portrayals of people that share similar traits. This can be anything from race to clothing style to area of study; as a species we are very keen on recognizing patterns and will therefore group those around us into categories based on what we know (or think we know) of others like them. This is exaggerated by a form a bias: we tend to more readily notice things that are different from ourselves and use them as labels.
I've had this in mind for a while, but it was recently brought to my attention by an encounter at work. I was walking down one of the hallways when I passed a janitor who happened to be black. Almost instantly the thought came into my head, "I wonder why all the janitors here are black?" Soon I was thinking about potential class and educational barriers and even - horrors! - inherent ability. It was at this point that I stopped myself. This train of thought couldn't be right, so I re-examined it and realized that I had taken for granted the fact that all the janitors were black. In reality, the janitors in our building are a fairly even mix of races; I had just noticed that the black ones were black because this was the trait most different from me. Whenever I would see one, rather than thinking "there's a janitor" like I did for any of the other races, I would think "there's a black janitor." While this was technically true and without any negative association, it was a distinction that was applied unevenly, resulting in my unquestioned assumption that all the janitors were black simply because that was the only descriptor that had ever been applied to janitors in my mind.
This has happened to me before. I've found myself moving from people like Ian McKellen and Neil Patrick Harris to thinking, "Man, all the great actors are gay!" In addition to this being a conclusion based on a terrible sample size, it failed to take into account people like Liam Neeson and Viggo Mortensen, other great actors for whom I had not associated a sexual orientation with their careers because it wasn't different from mine. For my mind, there was no reason to think, "Wow, they're a great actor and they're straight," because the fact of them being straight was considered to be something normal enough to be assumed and therefore not explicitly remembered. But when Ian McKellen came along, the knowledge of him being gay was so shocking (Gandalf is gay?!?!) that it was burned into my memory as being associated with his profession.
Now I'm not saying that we should all keep track of all aspects of a person, however familiar they may be - that would take far too much work. And not associating various traits of people to each other may very well be impossible. But whenever we find ourselves wondering why all of a certain demographic is a certain way, it would pay to examine our base assumptions. It's entirely possible we have accepted something false as truth simply because one aspect of it was more noticeable.