Throughout the book, Douglas emphasizes that behavior reflects personality, a theory that is the crux of criminal profiling. An offender’s behavior towards the victim and at the crime scene provides clues to the person’s personality. Behavior reflects specific aspects of an individual’s background and thought process that are unique to the offender. Behavior can also indicate whether or not there is a previous criminal history. It can also point to guilt or a lack thereof. While behavior can change and evolve over time, particularly if an offender is not caught for years, underlying personality traits are fixed and will be reflected in any new behavior.

Douglas postulates that behavior is not necessarily something that can be controlled—for instance, he coined the term ‘signature’ to describe a unique compulsion that a serial offender may have. The signature is not required for the completion of the crime; rather, it is a ritual or impulse that the offender must do to relieve the compulsion. A signature remains constant through all the cases involving a particular offender and reflects something in the offender’s personality that would compel him to repeat his crime over and over again. Another example of behavior that may be beyond conscious control is post-offense behavior—the memory of the crime may change behavior patterns, leading to changes like increased substance abuse, altered sleeping patterns, or decreased stress tolerance.