I often work with serial offenders and am quite interested in the techniques used by law enforcement to link different crimes across time to one offender. Authors John Douglas, Ann Burgess, Allen Burgess, and Robert Ressler in their text, The Crime Classification Manual: A Standard System for Investigating and Classifying Violent Crimes (2006), provide a fascinating, yet haunting discussion on the topic covering three of concepts used to identify serial criminals: Victimology, modus operandi and signature.
The victimology is simply the characteristics of the offender's victim. This may include the victims' age, gender, race, occupation, physical attractiveness, relationship status, and perceived vulnerability to name a few. Sometimes there is an identifiable likeness in victims chosen by serial offenders and sometimes there is not.
The Modus operandi, often referred to as MO, are specific actions taken by a perpetrator in order to complete the crime. They are a set of learned behaviors that are used because they work. An example of an MO for a serial rapist might be to hang out in shopping mall parking lots at night, forcing an unaccompanied woman into her car at gunpoint, and raping her. The MO may evolve over time as the criminal gets better, more efficient, or suffers set backs because of a particular method. The evolution of an MO is textbook operant conditioning, a form of learning. It describes a set of voluntarily learned behaviors that are reinforced or modified as a result positive or negative consequences. So, if the rapist is successful using the aforementioned strategies, he or she is likely to use them again. If not, lets say it took too long for the victim to unlock the car, he or she will modify the strategies accordingly. Thus, as most criminals attempt to improve, their MO will change gradually over time.
The signature, sometimes referred to as a "calling card", is another element of criminal behavior that occurs during the commission of a crime. It entails the aspects of the criminal behavior that go beyond those necessary for completion of the crime (the MO). Psychological traits, needs, and deviancy may be revealed via careful analysis of signature. Experts believe that criminal's fantasies are evident in their signature. If the fantasy is that of humiliation, for example, the criminal may pose his or her victims in degrading ways before or after they torture, kill, or rape them. Exemplified using the above hypothetical, the rapist could complete this signature by having the victim pose for photos or release her back to the parking lot nude after the commission of the crime. In either case, both acts are beyond those necessary to commit the crime of rape and both are replete with themes of humiliation. When the rapist offends again, the fantasy will be repeated and the degrading poses or release of a nude victim will be their signature.
Not all serial cases include a signature. Sometimes the offender will not be able to complete the signature aspects of their crime. Factors such as time, victim behavior, or unforeseen events like intrusions, can prevent them from completing the behaviors beyond those necessary to commit the crime. When this occurs, the offender is often less satisfied with crime.
Due to often unreliable victimology and evolving MO's, Douglass, Burgess, Burgess, and Ressler, place a great deal of significance on the signature as it tends to remain the same across time. Signature is apparently so valuable that the authors credit it with linking high profile cases such as those committed by serial rapist Ronnie Shelton, serial bomber, Ted Kaczynski, and serial killer Nathaniel Code.