In general, evidence of a defendant’s past misdeeds isn’t admissible in court. Part of the thinking is that a jury might be unfairly inclined to convict a defendant of the current charge if it learns that he committed similar acts in the past. Jurors might convict simply because they assume the defendant behaved in the way that he’s behaved in the past. Or they might just decide that, whether or not he’s guilty this time around, he deserves punishment. (See Can prosecutors present evidence of a defendant’s bad character?)
The rule against this kind of “predisposition” evidence has important limitations and exceptions. In many states, one critical exception has to do with prior instances of domestic violence. In these states, in a prosecution for domestic violence, prosecutors can often introduce evidence of the defendant’s previous acts of domestic violence. Before letting the evidence in, though, the judge may have to evaluate whether it would be fair to do so.
Example: Gus is on trial in California for domestic battery with corporal injury and misdemeanor assault. The prosecution alleges that he punched, head-butted, and choked his girlfriend. The prosecution wants to introduce evidence that Gus once, nine years ago, punched, slapped, and choked a former girlfriend. The judge decides to admit the evidence. An appeals court agrees with the judge’s decision. It finds that, even though the prior incident happened relatively long ago, the similarities in the incidents indicated Gus was “prone to violent rages, and engage[d] in punching and choking when in the throes of one.” The appeals court noted that there was no reason to believe that the former girlfriend had manufactured her story when the defendant came under suspicion for the current charges. It also found that the testimony about the prior incident was “no more inflammatory” than the testimony about the incident at hand. (People v. Morton, 159 Cal.App.4th 239 (2008).)