There are several reasons why those on the left think that the right is credulous and vice versa.
First, you are measuring credulousness against the backdrop of your own biases. If you’re on the left, you’re much more likely to believe, or perhaps not even pay attention to, conspiracy theories regarding Halliburton, Big Oil, the military, big business manipulating society, etc. Even a crazy conspiracy might not get much notice, because it’s still in the realm of things that confirm what you want to believe in the first place. So if someone tells you that, say, Blackwater gets its contracts because it’s an evil mercenary company that ‘disappears’ its rivals, you might think, “Hmm… I don’t know about that, but Blackwater is certainly an evil company that employs mercenaries, so I guess it’s possible.”
But you probably won’t think the person who told you that is a raving loon. You might think perhaps they’re going a little to far, but understandably so.
But if someone on the right comes up with a conspiracy theory that gores one of your oxen, not only are you more likely to take notice of it, but the fact that it is so far away from your own preconceived notions means you’re going to see it as completely crackpot.
This is basically a long-winded way of saying that confirmation bias plays a big part here.
There’s an example in this very thread - a poster earlier up made the flat-out statement that Republicans are less well educated than are Democrats. You probably didn’t even take notice of it, or if you did you might have just went, “Well, duh. Of course they are.” On the other hand, if I had popped in here and said, “Republicans aren’t credulous, because they have better educations than Democrats,” a lot of you probably would have jumped on me for it, or at least made note of yet another wacky Republican who believes things that aren’t true.
Except it’s true. Republicans tend to have better educations than Democrats - this is true at all levels - high school grads, 2 year college, 4 year college, and masters programs. The only place where Democrats have an advantage is at the Ph.D level, and that’s a very small sample of the population. In 2004, if you were a high school dropout, you’re more likely to vote for Kerry than Bush. If you had a college degree, you were more likely to vote for Bush. And it’s not a small gap, either. According to this table at Wikipedia, while comes from the American National Election Study, in 2004 21.9% of Democrats had college degrees, while 35% of Republicans did.
Does that suggest that Republicans are more credulous than Democrats?