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.