Matt Yglesias pulls this quote from a report on immigration:

In both experiments, the information influenced attitudes very little. We conclude by noting the potential limits of "information effects" on mass attitudes.
I think fair-minded people who make a living arguing opinions should always keep this in mind. Facts have limits. People will often believe what they judge to be in their interest to believe. This is  a fairly human trait--as in the mother whose son has just butchered a family and tells us that he was always "such a good boy." No one ever shoots anyone. The weapon magically discharges.

In these discussions of the Civil War and Ron Paul, I've had ample opportunities to reflect on "the potential limits of informational effects."