James v. Illinois

The Court felt that expanding the impeachment exception to all defense witnesses would have a chilling effect on a defendant’s ability to present his defense for three reasons. First, defense witnesses pose difficult challenges. Hostile witnesses called by the defense might willingly invite impeachment. Friendly defense witnesses might, through simple carelessness, subject themselves to impeachment. Also, expanding the impeachment exception to encompass the testimony of all defense witnesses would dissuade some defendants from calling witnesses who would otherwise offer probative evidence.

Second, the defendant rarely fears a perjury prosecution since the substantive charge is usually much more compelling. A witness other than the defendant fears a prosecution for perjury. Therefore, the Court’s need to deter perjured testimony is less than where the witness is the defendant, so that illegally obtained evidence can be introduced against them.

Third, expansion of the exception would significantly weaken the exclusionary rules’ deterrent effect on police misconduct by opening the door inadvertently to the admission of any illegally obtained evidence. This expansion would enhance the expected value to the prosecution of illegally obtained evidence by increasing the number of occasions when evidence could be used.