Mona Charen at NRO wonders whether Cain is being borked or is being evasive because there is something to this. Her thoughts are here, and I have little to add. Nor do I know much about the accusations and their likelihood of being true.
And that's a revealing limitation on my part. I am very likely to make my decision about whether Cain is guilty on the basis of how he responds to the accusations, not any basis of examining the facts. It's a shortcut we use a thousand times a year - does Jack seem to be lying? A great timesaver.
The weakness is rather apparent, though. It's a good rule of thumb, but not a perfect one. In particular, excellent liars exploit this human tendency by imitating innocent people, and distracting us with how terrible the accusers are. Bill Clinton was exceptionally good at this. He didn't act like a guilty person, and he didn't go over-the-top by trying to act like a completely innocent person. He acted just like a person who was guilty of some small, unimportant thing, but outraged that people would accuse him of something large. Of course he drove 6mph over the speed limit, but really...
Even after some large things turned out to be true.
And now here I am, doing what I complained about others doing, because it's a timesaver. I don't want to put in the energy to read up on this.