In most of these trials, juries acquitted the defendants on the grounds of insanity. The insanity plea afforded defendants an established defense on which to rely and permitted them to introduce proof of adultery or seduction where otherwise they could not have done so. While many contemporary observers have assumed that juries did not really believe that the defendants were insane but rather, acquitted them because of an unwritten law mandating the death of a libertine, an analysis of 10 such cases indicates that defense attorneys in many of these trials offered more than perfunctory testimony about the alleged insanity at the time of the killing. Some produced impressive expert testimony, citing specific symptoms of insanity. In addition, defense attorneys, judges, and medical experts devised certain definitions of legal insanity that expedited acquittals, including notions of temporary insanity, irresistible impulse, and moral insanity. Further, the puny efforts of the prosecutor in some of these cases also contributed to acquittals and increased the permissiveness of the insanity defense in cases involving the unwritten law. 28 footnotes.