In practice, the people I know who seem to make unusual efforts at rationality, are unusually honest, or, failing that, at least have unusually bad social skills.

It's the same "liar circuitry" that you're fighting, or indulging, in the internal or external case - that would be my second guess for why rational people tend to be honest people.

I have another alternate hypotheses: most normal people are such poor rationalists, that it simply isn't worth the effort to develop proper "lying skills" (if that's an acceptable term - yes, i know it sounds weird as a phrase!!). In a majority of the situations that rationalists find themselves in, normal people will do such a thoroughly good job at deceiving themselves, and convince themselves so thoroughly, that there isn't any point in trying anymore. For that matter, there is probably a reason why even seasoned con-men have set patterns.. the number of ways in which people(average joe/jane) can be manipulated, is not just finite, but also likely to be extremely limited.

Alternately with trained rationalists, the odds of successfully telling a lie are relatively and significantly smaller, so it serves as a tremendous pico-economic disincentive to even attempt telling a fabrication. This would also explain, perhaps in some ways better than the internal circuitry theory why there is a strong correlation between rationality and honesty.