I think it's more likely that, as people become more and more familiar with their own worldview it starts to feel sort of obvious to them. This is the "Curse of Knowledge". Once one feels like they know something they can no longer appreciate what it's like to not know it. Then it starts to feel as if people who disagree with them are all idiots or evil or both because it seems like they keep bleating on about how the sky isn't blue.
I understand what you're saying Naomi, but I think it is much more than that, and far less innocent in it's nature. If we step away from political or moral debates in general and talk about a disagreement say in a technical area (like software development, as an example), I've found that those people who the are the most knowledgable - especially those at the very top of their profession - are often the least likely to treat others as idiots, much less as evil. And instead of that "curse of knowledge" what I've most often seen is a kind of clarity in thought that makes it easier and more likely that their explanations cut right through the errors of the less knowledgable and without sarcasm or personal attacks. This isn't just an accident, but rather a matter of a benevolent confidence that comes with established competence and extensive knowledge. I would say that the 'innocent' progressives have simply spent their formative intellectual years in an environment steeped in progressive thought (the content) and were taught to respond to other views with sarcasm, ridicule and personal attacks (the method). And that it is a bit more than just an unemotional conditioned-reflex to respond in that fashion. It is also a defense of the pack against an outsider and against a threat to the entire way of thinking. It is an attack on something that might threaten the sense of superiority that was sold to the newly indoctrinated 'thinker'. Think about this... if the cause were as you say, "...because it seems they keep bleating on about how the sky isn't blue" then we would see instances of an argument appear in stages. The starting stage where positions are announced or become apparent. The next stage would be where each party tries to show that it's side is the better one to take, and then comes the stage where exasperation begins to show because the other side won't come around, and finally, we would see accusations of evil idiots coming from the progressives. But it doesn't work that way. The progressive arguments automatically start with a two-pronged nature: A politically correct position that is held as a moral absolute, and the assumption that the other side is made of evil idiots. You don't see a stage where reason is present - not in any sense of being open to hearing opposing logic - only the unwavering assumption that opponents are evil idiots.
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