Have you ever seen the movie Swimfan? The following is from the Wikipedia entry for stalking: Psychologists tend to group stalkers into two categories: psychotic and nonpsychotic. Many stalkers have pre-existing psychotic disorders such as delusional disorder, schizoaffective disorder, or schizophrenia. Most stalkers are nonpsychotic and exhibit disorders such as major depression, adjustment disorder, or substance dependence, as well as a variety of Axis II personality disorders, such as antisocial, avoidant, borderline, dependent, narcissistic, or paranoid. The non-psychotic stalkers' pursuit of victims can be influenced by various psychological factors, including anger and hostility, projection of blame, obsession, dependency, minimization and denial, and jealousy.

In "A Study of Stalkers," Mullen et al (2000) identify six types of stalkers:

• Rejected stalkers: pursue their victims in order to reverse, correct, or avenge a rejection (e.g. divorce, separation, termination).
• Resentful stalkers: pursue a vendetta because of a sense of grievance against the victims - motivated mainly by the desire to frighten and distress the victim.
• Intimacy seekers: The intimacy seeker seeks to establish an intimate, loving relationship with their victim. To them, the victim is a long sought-after soul mate, and they were meant to be together.
• Eroto-manic stalker: This stalker believes that the victim is in love with them. The erotomaniac reinterprets what their victim says and does to support the delusion, and is convinced that the imagined romance will eventually become a permanent union. They often target a celebrity or a person of a higher social status (though it is important to note, not all celebrity stalkers are erotomaniacs).
• Incompetent suitor: despite poor social/courting skills, possess a sense of entitlement to an intimate relationship with those who have attracted their amorous interest.
• Predatory stalker: spy on the victim in to prepare and plan an attack - usually sexual - on the victim.

The 2002 National Victim Association Academy define an additional form of stalking. The Terrorism stalker also known as the political stalker, uses stalking as a means to accomplish a political agenda, often by using threats and intimidation to force their target to refrain and/or become involved in some particular activity, regardless of the victim's consent.

Many stalkers fit categories with paranoid disorders. Intimacy-seeking stalkers often have delusional disorders that are secondary to preexisting psychotic disorders such as schizophrenia. With rejected stalkers, the continual clinging to a relationship of an inadequate or dependent person couples with the entitlement of the narcissistic personality, and the persistent jealousy of the paranoid personality. In contrast, resentful stalkers demonstrate an almost - pure culture of persecution, - with delusional disorders of the paranoid type, paranoid personalities, and paranoid schizophrenia.

Gender Studies in Stalking Pathology

Most stalkers are male, but women can also be stalkers. The demographic characteristics of the male and female stalkers do not differ, although more male stalkers report a history of criminal offenses and more report substance abuse. The psychiatric status of male and female stalkers do not otherwise differ. The duration of the time invested in stalking and the frequency of associated violence were equivalent between male and female stalkers. Women are more likely to target someone they have known such as a professional contact, and are more likely to target other females. Men, on the other hand, do not usually target other men. Women are also much less likely to target a stranger.

In "A Study of Women Who Stalk", by Rosemary Purcell and Michele Pathé, the authors concluded that the two major psychiatric variables that differentiate female from male stalkers is motivation for stalking and choice of victim. Female stalkers seek intimacy with the victim, who usually is someone already known. The victim is most often chosen from those who assume a professional role of helper. This could be a doctor or nurse, a therapist or counselor. Context was found to differ, but the conclusion was the intrusiveness and harmfulness did not. In other words, female stalkers are potentially as dangerous as any male stalker. A Study of Women Who Stalk. AJP 2001