Smart people don't think others are stupid merely because of the beliefs they hold. That is, being a [Southerner, Northerner, Republican, Democrat, Indian, American, Christian, Muslim, Atheist, Tauist, Piist, vi user, emacs user, ...] does not mean a person is stupid. There are smart people who really have thought things through and ended up at each of those positions for good reasons.
But smart people might conclude some specific individual is stupid because of consistently erroneous patterns of thought and analysis which do not improve with experience. There are a few individuals who I've spoken to on a variety of topics over the course of years who, as a rule, develop beliefs based on sketchy evidence and then retain those beliefs even in the face of mountains of contrary evidence. Even when they hold essentially the same views as I do, it's often for bad reasons; there isn't really a coherent thought process or a body of evidence that got them to that point. Such people really are stupid.
Even when they hold essentially the same views as I do, it's often for bad reasons
I noticed this too, and for a long time it bothered me. But I recently read Jonathan Haidt's book The Righteous Mind, and in it he points out that a lot of people appear to hold believes about a wide array of issues (politics, religion, consumer products, and so forth) that they don't really hold based on logic and evidence, but to signal group identification and affiliation.
In addition, he points out that, on a wide array of issues, people tend to have gut, intuition-based reactions first, then look for evidence to support their intuition, while a lot of us assume or want to assume that it works the other way around.
To your first point, some scientist believe it is hard wired in our DNA to associate with any form of group (as this played a role in evolution).
Here's a related interesting talk on TED that I recently saw:
http://www.ted.com/talks/lang/en/jonathan_haidt_on_the_moral...
The real question is what to do with stupid people?
I usually try to hold it as their defense. Something like: "It's OK he said something like that because he's -- well, like that. Now let's change the subject quickly." This makes me look like a bad person but hey, it's better than pick on people because they didn't use their brain.
And then I try to avoid stupid people. There is only so much time.
But smart people might conclude some specific individual is stupid because of consistently erroneous patterns of thought and analysis which do not improve with experience. There are a few individuals who I've spoken to on a variety of topics over the course of years who, as a rule, develop beliefs based on sketchy evidence and then retain those beliefs even in the face of mountains of contrary evidence. Even when they hold essentially the same views as I do, it's often for bad reasons; there isn't really a coherent thought process or a body of evidence that got them to that point. Such people really are stupid.