I love Jack McCoy.
Fun Fact: He is the only DA tough enough to convict Chuck Norris.
When he was an ADA on Law and Order (side note: The original is the only watchable version. You have a right to like the spin-offs, but you are wrong in that right.) McCoy will sometimes call on a psychologist to take the stand against a defendant. When no insanity plea is used by the defendant, the psychologist is being used as a criminal profiler. The profiler is giving his expert opinion as to the personality and thought process of the criminal, maybe even describing some phsyical features.
The problem is that criminal profiling tends to be subject to the P.T. Barnum effect. "The P.T. Barnum effect is the tendency of people to accept high base rate descriptions-descriptions that apply to almost everyone-as accurate" (Lilienfeld, 574). In a courtroom, the jury wouldn't buy a vague description of a criminal as "angry" or "disturbed", because most people could venture a guess that someone who acts outside the law as being "angry" or "disturbed".
In all reality, criminal profiling is at best a guess. A study by Homant & Kennedy in 1998 (Lilienfeld, 575) agrees. The study showed that professional profilers were no better at distinguishing peronsality traits of murderers than untrained, college students. As other studies have replicated these findings, it is strange that criminal organizations still train profilers.
Even if his criminal profiler is a bust, Jack McCoy will still win the case. (Dun Dun!)