The classic study of the American jury shows that when a defendant's criminal record is known and the prosecution's case has weaknesses, the defendant's chances of acquittal are thirty-eight percent, compared to sixty-five percent otherwise. Because of the danger that jurors will assume that the defendant is guilty based on proof that his bad character predisposes him to an act of crime, the courts and legislatures have attempted to circumscribe the use of such evidence. Some prosecutors, however, although well aware of the insidious effect such prejudicial evidence can have on jurors, violate the rules of evidence, as well as ethical standards, by deliberately introducing inadmissible evidence in order to obtain a conviction, despite the risks involved.
Bennett L. Gershman, Proving the Defendant's Bad Character, 11 Am. J. Trial Advoc. 477 (1988), http://digitalcommons.pace.edu/lawfaculty/269/.