Acording to the psychological cognitive dissonance theory, people interpret evidence in ways that justify their own past actions. For example, suppose you voted for George W Bush in 2000. When updating your beliefs about W's performance, you may put more emphasis on positive pieces of information that justifies this past vote, and less emphasis on negative evidence that makes the past vote look bad. When thinking about the war in Iraq, for example, you may put more weight on the positive effects of the surge, rather than the negative effects of war in the first place.
To test this theory, Mullainathan and Washington compare the presidential opinion ratings of people who turned 18 in time to vote in the presidential election, with the opinion ratings of similar individuals who were not quite 18 in time to vote. Their results support the cognitive dissonance theory. From the abstract:
We examine the presidential opinion ratings of voting-age eligibles and ineligibles two years after the president’s election. We find that eligibles show two to three times greater polarization of opinions than comparable ineligibles. We find smaller effects when we compare polarization in opinions of senators elected during high turnout presidential campaign years with senators elected during nonpresidential campaign years.Read the paper.