My favorite study (pdf) in this space was by Yale’s Geoffrey Cohen. He had a control group of liberals and conservatives look at a generous welfare reform proposal and a harsh welfare reform proposal. As expected, liberals preferred the generous plan and conservatives favored the more stringent option. Then he had another group of liberals and conservatives look at the same plans, but this time, the plans were associated with parties.
Both liberals and conservatives followed their parties, even when their parties disagreed with their preferences. So when Democrats were said to favor the stringent welfare reform, for example, liberals went right along. Three scary sentences from the piece: “When reference group information was available, participants gave no weight to objective policy content, and instead assumed the position of their group as their own. This effect was as strong among people who were knowledgeable about welfare as it was among people who were not. Finally, participants persisted in the belief that they had formed their attitude autonomously even in the two group information conditions where they had not.”
I agree with Ezra that this is a big part of the story. Once we've decided that we belong to a party or a religion or like a certain sports team, our brains tend to follow along with what the other people that belong to those same groups think. As a Dolphins fan I tend to agree with my fellow fans that the Jets are a loathsome football team. If two studies came out, one saying it proves the Jets are loathsome and the other saying it proves they aren't, I would probably be inclined to believe the former.
These tribal impulses are strong and they seem to extend into all walks of life. The problem when it happens in politics is that it prevents good policy from being made. So we need to be constantly aware of the phenomenon so that we can prevent ourselves from falling into its traps.