Also, I highly recommend Malcolm Gladwell's new book, Talking to Strangers.
If you've ever wondered why people get away with stuff for so long, this book's explains how.
Basic premise is that "most" people are willing to believe that the world is mostly good, that people are generally honest, that they can tell when someone is telling a lie. This makes sense in evolutionary terms, because if our default wasn't to trust, then we would spend too much time getting people to prove themselves and spending energy on checking people out, rather than just cooperating.
Gladwell argues that it's worth the small price of being conned every now and again, because our lives are much easier being able to trust other people most of the time.
However, those that have little trust in others, again often something that can be traced back to a difficult childhoods, are very useful in professions such as fraud detection. Their default is that everyone is lying, so they look for the lies. They don't do what most of us do and allow ourselves to be fooled into thinking everything is OK, because our default is that people are mostly truthful.
One last thing on this book. Gladwell demonstrates that we expect people to show facial expressions that match the emotions that they have. So if someone "looks" remorseful, we allow ourselves to believe that they "are" remorseful. We don't like to think the person is deliberately looking remorseful because it will get them out of trouble.
I would say that the thing that Gladwell misses in this is instinct. How often do we see written on this board "something felt wrong" or "something about them" felt wrong. Gladwell sticks to us reading facial expressions, but humans communicate in much more complex ways than that. I think The people who get groomed by conmen or taken in by liars, are people who habitually override their gut feelings.