This differs from the MO (modus operandi), which means "a method of operating." The MO refers to behaviors of an offender for the purpose of successfully completing an offense. For example, an MO might be entering through a broken window.
The signature, on the other hand, is a behavior committed by an offender that is not necessary to complete the offense. Signature behaviors are repeated at each crime scene and include rituals, torture, mutilation, displaying the body in a shocking position, or using specific types of binding or weapons.
The profiler can gain insight into the motive by studying the MO and the signature. John Douglas explains the difference: "Modus operandi is learned behavior. It's what the perpetrator does to commit the crime. It is dynamic - that is, it can change. Signature ... is what the perpetrator has to do to fulfill himself. It is static; it does not change." The signature is part of the offender's fantasy. So while the MO may change as the offender becomes more experianced and sophisticated, his signature will not.
The offender's signature reveals a great deal about him. For example, a man enters a house while the woman who lives there alone is sleeping. He removes all of the phone cords before waking her and then gags her with one of her own socks. This would be an MO - he is preventing her from calling for help should she get an opportunity. The man then forces the woman to light six red candles. He drips hot wax on her before killing her. This would be his signature. If another crime scene shows evidence of red candles and wax drippings on the victim, it's very likely the authorities are dealing with the same perpetrator. If they also discover the phone cords removed and the victim gagged with a sock, the chances are even higher. A signature will not show up in every case. Not all perpetrators have one. If the offender is interrupted, the victim behaved differently than anticipated, or the body has been left for some time, the signature may not be there or difficult to locate.
Stay tuned for Part V, as I discuss the FBI's technique of developing a profile.
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