This is an interesting article from Psychology Today, via Bruce Schneier. It explains how con artists exploit our trust in part by showing that they trust us, and they appear to be making themselves vulnerable. The author describes THOMAS, The Human Oxytocin Mediated Attachment System, as a key brain circuit involved in trust decisions.
The key to a con is not that you trust the conman, but that he shows he trusts you. Conmen ply their trade by appearing fragile or needing help, by seeming vulnerable. Because of THOMAS, the human brain makes us feel good when we help others–this is the basis for attachment to family and friends and cooperation with strangers. “I need your help” is a potent stimulus for action.