Have you actually tested them for rationalizing? My own beliefs are that it's more likely to run into someone who rationalizes so much they are blind to their own rationalizing (and so can't recall any) than someone who is inhumanly honest.

(Tests in this case would include checking for hindsight bias, which is classic rationalizing, and having them do that test on YourMorals whose name I forget where you're given two summaries of studies for and against gun control and asked to criticize them - usually showing imbalance towards your favored side. But you're a LWer, I'm sure you can think of other tests.)