Fad social science? Sure, that sucks, but even "forensic science" evidence is often questionable (http://www.innocenceproject.org/understand/Unreliable-Limite...). Even worse, even if the test really does work (e.g. DNA testing), we get abuse of statistics like http://en.wikipedia.org/wiki/Prosecutor%27s_fallacy - "the probability of a random guy having this weight of evidence against him is 1/1 000 000, so this guy must be guilty!" (of course, there are 300 Americans you could convict of any one crime under this standard...)

Let's just say that modern justice systems are not without their faults.