If you’ve watched any crime or courtroom dramas on TV, you’ve probably picked up on some police and legal jargon. Some of these terms are easy to figure out, but others can be more challenging, especially when they’re not explained in the show. For example, one commonly used term is “M.O.” Detectives investigating a crime or lawyers building a case may refer to a criminal’s M.O. as other characters nod knowingly. But what is an M.O., and why does it matter? 

Modus Operandi

When referring to crime, M.O. is an abbreviation for the Latin phrase modus operandi, which means “mode of operating” in English. Police use this term when a perpetrator has a distinct, recognizable way of committing crimes. This distinctive pattern helps police track serial offenders, as they can sometimes assume that separate crimes committed in the same way are likely to be committed by the same person. 

The term modus operandi doesn’t exclusively apply to criminal cases, either. Usage of the term has existed in English as far back as the 1600s in a number of contexts, although the common abbreviation to M.O. only dates to the mid-1950s. For example, M.O. is often used by businesses in regard to analyzing their competitors' moves. 

Use in Criminal Law

While most frequently used by investigators to track down criminals, M.O. can be an important element of courtroom proceedings as well. According to the Federal Rules of Evidence, modus operandi can be used as the basis for admitting evidence of other crimes in criminal cases. 

Unlike in police work, where M.O. can be used freely as a guide for their investigation, there are specific rules about when it can be accepted as evidence in court. The law states that evidence of a person’s character is not proof that they acted in accordance with that trait. In essence, just because they are likely to do something doesn’t prove that they did it.

With certain exceptions, however, evidence of pertinent traits (in this case, the person’s M.O.), can be admissible to establish motive, opportunity, intent, preparation, plan, knowledge, and other elements of a prosecutor’s case. M.O. can also be used by defendants to show either their own or the victim’s traits.

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