In the wake of birtherism, people are talking about the anti-rationalism of the right (since we all know that everyone else is completely immune). I have seen a variety of reasons for anti-rationalism, from intellectual laziness to the effects of religion to an emotional need to believe nonsense. While some people either cannot or will not think rationally about their pet issues no matter what, I believe that many other people do not pursue rational inquiry about such issues simply because no one has told them that it is an option.
For example, in the public schools that I attended, we studied formal logic for all of one grading period, and even then only in the context of geometry. No one pointed out any wider applicability, least of all the history teacher whose answer to every challenge was to restate his original assertion more loudly.
Later, when I did volunteer work in HIV education, I sometimes had to step callers through the process of understanding relative risks. I quickly realized that I was the first person to explain to them how to do so, and I could almost hear the light bulbs going on over their heads when they got it.