The "mercy rule" allows defendants in criminal cases to offer evidence of their good character as a defense to the charges. (See, for example, Federal Rule of Evidence 404; Cal. Evidence Code, § 1102.) But there are limitations to the rule, and sometimes, taking advantage of it can backfire.
Defendants are allowed to present evidence of their good character for some reasons, but not others.
For example, evidence of good character isn't allowed for the purpose of arguing that the defendant committed the crime but shouldn't be convicted. In that way, the so-called "mercy" rule isn't really about mercy at all, because defendants aren't allowed to use it to ask for leniency or forgiveness.
But defendants can present evidence of good character if it tends to show that they didn't likely commit the alleged crime(s). For example, if the defendant is charged with fraud or embezzlement, they can offer evidence that they're honest and law-abiding: Such evidence relates to the theory that honest, law-abiding people are less likely to steal money than people without this character trait.
When the defendant wants to present evidence of good character, they normally call character witnesses. A character witness is simply someone who knows the defendant well and testifies about the defendant's good character. Character witnesses are often family members, friends, and sometimes employers, coworkers, or others.
Though the rules of evidence vary from state to state, a character witness's testimony is usually limited to their opinion of the defendant and knowledge of the defendant's reputation. They might say that in their opinion, the defendant is honest and has a reputation for being truthful, for example.
Like all other evidence, in order to be admissible, character evidence must be based on personal knowledge and be relevant. Character evidence is relevant if it relates to a trait of the defendant's character that has some connection to the charged crime.
Norman Bates is charged with assaulting Roseanne Fell. Bates calls to the stand a close friend to testify, "In my opinion, Bates is a nonviolent person who wouldn't hurt a fly." This kind of character evidence would be allowed because it suggests Bates has a tendency to be peaceful, making it less likely that he assaulted Fell.
But Bates could not call his friend to testify that he is honest or hard-working, because those character traits aren't related to the crime of assault.
Presenting evidence of good character comes with a lot of risks, which is why defendants don't often take advantage of the mercy rule.
Here are just some of the risks involved:
If the defendant doesn't offer evidence of his good character, the prosecutor typically can't offer evidence of his bad character. Evidence rules generally forbid prosecutors from attacking a defendant's character unless the defendant first "opens the door" by presenting evidence of good character. Character evidence is barred in this situation because it is too harmful. A judge or jury might convict a defendant for being a "bad person," or might assume that the defendant acted in conformity with his "bad character" during the events in question, even if the evidence of guilt is otherwise weak.
Character Evidence in Sexual Assault Crimes
A perceived increase in sexual assault crimes, such as rape and child molestation, has alarmed legislatures in many states. They believe that those who commit sexual assaults tend to repeat their crimes, and that, especially when the victims are children, the crimes are difficult to prove in court. As a result, many states have created special exceptions to the character evidence rules for sexual assault prosecutions.
The exceptions allow prosecutors to offer evidence that defendants charged with sexual assault or child molestation have committed those crimes in the past. (Many jurisdictions have equivalent rules for domestic violence prosecutions, allowing admission of past domestic violence to prove the charged offenses.)
The exception for past acts of child molestation was a major factor in the highly publicized 2005 trial of late singer Michael Jackson. Jackson was accused of molesting a teenage boy at his Neverland ranch. The judge allowed the prosecutor to offer evidence that he had previously molested a number of other young boys, even though he wasn't charged with molesting them. The lurid details of the previous accusations consumed many more days of the trial than did the testimony that dealt with the charged crimes. Jackson was nevertheless acquitted of all charges.
Getting Around the Ban on Evidence of Bad Character
Despite the rule barring evidence of a defendant's bad character when that character hasn't been put in question, prosecutors routinely argue that "bad person" evidence is relevant for another reason. For example, a judge would probably allow a prosecutor in an assault trial to present evidence that the defendant previously assaulted the same victim. In that situation, a judge would likely rule that the evidence isn't about the defendant's character: Instead, it's evidence that the defendant had a problem with the victim, which tends to show the defendant had intent (and possibly motive) to assault them.
Another non-character theory that prosecutors sometimes use to present evidence of a defendant's past bad acts is modus operandi, or "M.O." Under this theory, the prosecutor can offer evidence that the method a defendant used to commit past misdeeds is unique and nearly identical to the method allegedly used to commit the charged crime. Evidence of the prior misconduct is then admissible—not to paint the defendant as a bad person—but to show that the common M.O. points to the defendant as the perpetrator of the charged crime.