Researchers at the University of San Francisco did a study to determine if particular groups of people were better able to detect a lie than others. Out of a selection of various law-enforcement and intelligence officers, lawyers, psychiatrists, and many others, only one group showed a better-than-random ability to detect a liar: the Secret Service. Why? Among their speculations:
The members of the Forensic Services Division of the Secret Service whom we tested spend part of their time interrogating people who threaten to harm government officials. Secret Service officials told us that most of these people are telling the truth when they claim that their threat was braggadocio, not serious. It is only the rare individual who is lying in his or her denial and actually intends to carry out such a threat. Members of the criminal justice community told an opposite story; they believe that everyone lies to them.
So in order to catch a liar, you have to be willing to believe that most people are telling the truth.
(Thanks to erudito for the link).