In the beginning, Dennis Rader's neighbors referred to him as an "ordinary, unassuming man." To his family, the words "soft spoken" and "family man" were all words perfect in description. After a shocking investigation, however, both family and friends of Dennis Rader would come to realize that this ordinary member of society was also classified as a notorious psychopath, often referred to as the "BTK" because he binded, tortured, and killed his victims. Seemingly identical to the majority of the population, Dennis Rader and other serial killers like him, differ substantially in certain physiological functions. In an interviewwith the infamous BTK killer, the killer refers to the drive inside of him to murder as the "Factor X." He describes it as a feeling of a loss of control; he thinks he might have been dropped on his head as a child because the feelings inside of him are unnatural. Unintentionally, the BTK killer is suggesting that there is indeed a biological difference between the psychopathic brain and a typical brain. This idea is supported in an article written in Crime Times. The article suggests that most psychopaths differ in their reactions to fear-provoking stimuli. In addition, the fear-potentiated startle reflexes of psychopathic individuals is minor in comparison to non-psychopathic. Personality traits such as self-confidence, impulsiveness, and aggressiveness also play a crucial role in the make-up of a psychopath. So while Dennis Rader, may seem like a calm and collected individual, the "Factor X" inside of him may actually exist.