The author of this book, Petherick, states that the signature of a crime is different from the M.O., or modus operandi of a crime. The M.O. refers to the behaviors needed to carry out the crime. On the other hand, Petherick defines the signature as "the expression of the underlying needs of the offender" (page 129). The signature can be a way to connect different crimes and to understand the psychology of the perpetrator. The M.O. can be a way to develop a profile of the offender.
The M.O. and the signature can overlap at times. As the author states, quoting Turvey (1999), "signature and M.O. needs may be satisfied by the same behavior" (page 129). For example, the author gives the example of an offender wearing a mask. The wearing of the mask is both the M.O. and the signature for this crime, though which mask the offender wears may change for each crime. The author states that the signature remains constant, though signature behaviors may change over time, but that the M.O. changes for each crime.
The signature is used to connect perpetrators to different crimes. For example, the author provides the example of a cluster of M.O. behaviors that linked two crimes that might have been committed by the same person--one crime in New Jersey and a later crime in Maine. In addition, the signature, or the ritualistic aspects of the two crimes were the same, including bites on the two victims in the same areas of their bodies. This evidence was ultimately not allowed in court, and the author states that this type of evidence can be useful in drawing connections between two cases in the investigative part of a crime but may not meet the threshold of being admissible evidence in a court of law (page 137).