Rational ignorance is an economics term occasionally borrowed by other disciplines. When the effort required to obtain correct information is just not worth your time, you are being rationally ignorant in not spending the time to get it. Ten hours of research to save yourself a dollar is just not worth it. When confronted with a complicated matter with little practical effect, people shrug and pick whatever pleases them on impulse. If you are choosing between two car rentals, one of which has a low rate plus a mileage charge, the other a higher rate with unlimited miles, most people do a quick estimate to see if there is going to be some large difference between them. If it is established that the two are going to turn out to be pretty close for your trip, it becomes silly for you to obsessively calculate whether your hotel is 3.9 or 4.2 miles from the airport, etc. You pick one. You are being rationally ignorant.
This plays out in politics because your individual vote has virtually no effect on the outcome of an election. Even in the 2000 Florida election or the 1970's election in NH decide by only a dozen votes, one vote doesn't matter much. If you personally had not shown up at the polls, same candidate would have been elected by eleven votes, or thirteen. Only in the rarest of situations does your vote matter. In such a situation, why not adopt the beliefs that make you more popular, or that make you feel better? You get nothing for your vote, but you might get something personal or social for being a person who votes a certain way, or seeing yourself as a person who votes a certain way. There are interesting discussions of this by Arnold Kling and Bryan Caplan, author of The Myth of the Rational Voter.
People who do vote do some minor exaggerating of their eventual impact on the race, but for the most part, know that their votes are being cast into the sea. We vote for the sense of participation, for an almost mystical connection to a set of ideas, to a candidate, or to the American ideal of citizen government. We may secretly hope that someday our one vote will be a decider, but even knowing the futility - even when we are a Democrat in Utah or a Republican in DC - we want to make a certain statement. To the world, to ourselves, to God, whatever; but a statement.
I have railed against the social cachet that comes from being a progressive in certain circles, and how miserable and shallow an excuse that is for adopting a set of views. Well, why not adopt the views that will make you feel better or more popular? Hmm?