Can a witness vouch for another's credibility?

We can't agree. One witness's vouching for another's credibility isn't remarkably reliable, but it is somewhat probative: The jury system itself is based in part on the notion that people can tell from someone's demeanor whether he's telling the truth. See, e.g., Rosales-Lopez v. United States, 451 U.S. 182, 188 (1981) (factfinders should base credibility judgments in part on demeanor evidence). Moreover, such vouching isn't especially pre-judicial. We think most people realize how hard it is to tell that someone's lying, and take witnesses' assurances of others' veracity with a good bit of salt.