Modus operandi, in a legal sense, can be best translated as a way of doing something. It is also called an M.O., and most people may be familiar with this term from the many legal or police dramas, where it is used repeatedly. Typically, modus operandi in legalese refers to a similar pattern of doing things when someone is suspected of committing multiple crimes. If the suspect has committed the crimes in the same way, it becomes easier to prove all of them, even if the only link between some of the crimes is the way they were committed. Individual jurisdictions determine specific evidence required to charge people with auxiliary crimes, and a strong M.O. argument is not always sufficient evidence, but it certainly may be evidence enough to further investigate a crime and get warrants or other permissions to question, search or seize property to make a case.
Probably the most common examples of modus operandi are used when serial killers are investigated. Many of them have specific habitual behaviors that they will employ when committing crimes. In essence they leave a trail of hints that they’ve committed the same type of crime through highly ritualized behavior. This is how criminal investigators are able to link one crime to another, and in doing this, they may also be able to cast the net further, looking at other criminal acts that appear to be similar and that might warrant investigation. Alternately if the person has not been caught, determining modus operandi may lead to clues about the killer’s identity.
Even when crimes are not so severe, trying to discover modus operandi can be useful in apprehending a criminal, getting arrest warrants, or prosecuting a case. The average criminal who is not a serial killer is still likely to do some things the same way, even if he or she tries to cover it up. For instance, in the movie Home Alone, one of the thieves insists on flooding the sink each time he robs a house. This little “way of doing things,” connects him and his partner to a number of crimes where the sink is intentionally flooded in a house that has been burglarized. M.O. is not always so straightforward or purposeful, but analysis of evidence may discover patterns previously unnoticed.
It is not always easy to prove modus operandi because behavior discovered must be unique. Saying, “Well, he carried a gun when he robbed the bank,” isn’t uncommon and wouldn’t be much of a strong M.O. On the other hand, if the criminal carried a gun while sporting a rubber duck on his head, the M.O. gets a little more interesting. Usually it takes several distinctive properties to make a strong case that a person may have committed a number of crimes of the same nature.