They can be admitted if the defendant "opens the door." That is to say that if a defendant offers testimony (i.e. he states that he was never in trouble his whole life.)that can be impeached, the prior bad acts can be introduced for the limited purpose of refuting his testimony, and not as evidence to establish that he commited th crime, for which he is presently on trial.

Prior bad acts of defendants and other witnesses are generally excluded as they lack relevance, provide little probative value, and create a risk of unduly prejudicing the defendant.