Maybe I'm just cynical but I think people vastly overestimate their own goodness. Often "goodness" is just a way to dress up powerlessness. Like an overweight man might say he's "stocky" or an overweight woman might say she's "curvy," so an undesirable or shy man or woman might emphasize the upside: "I would never cheat." There's a version of the typical mind fallacy in there: a person might genuinely think they would never cheat but be extrapolating from a position where the opportunity rarely presents itself. We can all talk about how, if we were in a position of political power, we'd never succumb to bribes or cronyism because we don't have any political power. It both makes us look good and, as far as we know, it's true. I think testimony, especially when it comes to ones moral worth, is the least valuable form of data available.