Stalking is a term commonly used to refer to unwanted attention by individuals (and sometimes groups of people) to others. Stalking behaviors are related to harassment and intimidation. The word "stalking" is used, with some differing meanings, in psychology and psychiatry and also in some legal jurisdictions as a term for a criminal offence. It may also be used to refer to criminal offences or civil wrongs that include conduct which some people consider to be stalking, such as those described in law as "harassment" or similar terms.
The difficulties associated with precisely defining this term (or defining it at all) are well documented. It seems to have been first applied to the harassment (in a general sense) of celebrities by strangers who were described as being obsessed. This use of the word appears to have been coined by the tabloid press in the United States.
Stalking can be defined as the willful and repeated following, watching, and / or harassing of another person. Most of the time, the purpose of stalking is to attempt to force a relationship with someone who is unwilling or otherwise unavailable. Unlike other crimes, which usually involve one act, stalking is a series of actions that occur over a period of time. Although stalking is illegal, the actions that contribute to stalking are usually legal, such as gathering information, calling someone on the phone, sending gifts, emailing or instant messaging. Such actions by themselves are not usually abusive, but can become abusive when frequently repeated over time. 
People characterized as stalkers may have a mistaken belief that another person loves them (erotomania), or have a desire to help the other person. Stalking consists of a series of actions which in themselves can be legal, such as calling on the phone, sending gifts, or sending emails.
Stalkers may use threats and violence to frighten their victims. They may also engage in vandalism and property damage and make physical attacks that are mostly meant to frighten. Less common are sexual assaults.
Stalking can be a terrifying experience for victims, placing them at risk of psychological trauma and physical harm. Common emotional consequences include depression, anxiety, loss of self-esteem, shame, hopelessness and a sense of vulnerability that can persist long after the stalking ends. It is common for victims to blame themselves (self-blame), especially if the stalking results from an established relationship with the stalker. Families and friends may contribute to this sense that the victim is at fault (victim blaming). Disruptions in daily life necessary to escape the stalker, including changes in employment, residence and phone numbers, may take a toll on the victim's well-being and lead to a sense of isolation.
According to Lamber Royakkers, "Stalking is a form of mental assault, in which the perpetrator repeatedly, unwontedly, and disruptively breaks into the life-world of the victim, with whom they have no relationship (or no longer have). Moreover, the separated acts that make up the intrusion cannot by themselves cause the mental abuse, but do taken together (cumulative effect)."
According to one study, women often target other women, whereas men generally stalk women only. However, a January 2009 report from the Department of Justice in the United States reports that "Males were as likely to report being stalked by a male as a female offender. Forty-three percent of male stalking victims stated that the offender was female, while 41% of male victims stated that the offender was another male. Female victims of stalking were significantly more likely to be stalked by a male (67%) rather than a female (24%) offender." This report provides considerable data by gender and race about both stalking and harassment.
Psychologists often group individuals who stalk 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 may exhibit disorders or neuroses 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). Some of the symptoms of "obsessing" over a person is part of obsessive compulsive personality disorder. The nonpsychotic stalkers' pursuit of victims can be influenced by various psychological factors, including anger, hostility, projection of blame, obsession, dependency, minimization, denial, and jealousy. Conversely, as is more commonly the case, the stalker has no antipathic feelings towards the victim, but simply a longing that cannot be fulfilled due to either in their personality or their society's norms.
The 2002 National Victim Association Academy defines an additional form of stalking: The 'Vengeance/Terrorist stalker'. Both the Vengeance stalker and Terrorist stalker (the latter sometimes called the political stalker) do not, in contrast with some of the aforementioned types of stalkers, seek a personal relationship with their victims but rather force them to emit a certain response favourable to the stalker. While the vengeance stalker's motive is "to get even" with the other person whom he/she perceives has done some wrong to them (i.e, an employee who believes is fired without justification from their job by their superior), the political stalker intends to accomplish a political agenda, also using threats and intimidation to force his/her target to refrain and/or become involved in some particular activity, regardless of the victim’s consent.
Many stalkers fit categories with paranoia disorders. Intimacy-seeking stalkers often have delusional disorders involving erotomanic delusions. 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.
One of the uncertainties in understanding the origins of stalking is that the concept is now widely understood in terms of specific behaviors which are found to be offensive and/or illegal. As discussed above, these specific (apparently stalking) behaviors may have multiple motivations.
In addition, the personality characteristics that are often discussed as antecedent to stalking may also produce behavior that is not stalking as conventionally defined. Some research suggests there is a spectrum of what might be called "obsessed following behavior." People who complain obsessively and for years, about a perceived wrong or wrong-doer, when no one else can perceive the injury—and people who cannot or will not "let go" of a person or a place or an idea—comprise a wider group of persons that may be problematic in ways that seem similar to stalking. Some of these people get extruded from their organizations—they may get hospitalized or fired or let go if their behavior is defined in terms of illegal stalking. But many others do good or even excellent work in their organizations and appear to have just one focus of tenacious obsession.
According to a U.S. Department of Justice special report a significant number of people reporting stalking incidents claim that they had been stalked by more than one person, with 18.2% reporting that they were stalked by two people, 13.1% reporting that they had been stalked by three or more.
According to a paper by Sheridan and Boon , in 5% of the cases they studied there was more than one stalker, and 40% of the victims said that friends or family of their stalker had also been involved. In 15% of cases, the victim was unaware of any reason for the harassment.
In 1999, Pathe, Mullen and Purcell wrote that popular interest in stalking was promoting false claims. In 2004, Sheridan and Blaauw said that they estimated that 11.5% of claims in a sample of 357 reported claims of stalking were false.
According to Sheridan and Blaauw, 70% of false stalking reports were made by people suffering from delusions of persecution. Another study estimated the proportion of false reports that were due to delusions as 64%.
According to a study conducted by Purcell, Pathé and Mullen (2006), 23% of the Australian population reported having been stalked.
Stieger, Burger and Schild conducted a survey in Austria, revealing a lifetime prevalence of 11% (women: 17%, men: 3%). Further results include: 86% of stalking victims were female, 81% of the stalkers were male. Women were mainly stalked by men (88%) while men were almost equally stalked by men and women (60% male stalkers). 19% of the stalking victims reported that they were still being stalked at the time of study participation (point prevalence rate: 2%). To 70% of the victims, the stalker was known, being a prior intimate partner in 40%, a friend or acquaintance in 23% and a colleague at work in 13% of cases. As a consequence, 72% of the victims reported having changed their lifestyle. 52% of former and ongoing stalking victims reported suffering from a currently impaired (pathological) psychological well-being. There was no significant difference between the incidence of stalking in rural and urban areas.
Budd and Mattinson found a lifetime prevalence of 12% in England and Wales (12% overall, 16% female, 7% males).
Dressing, Kuehner and Gass conducted a representative survey in a middle-sized German city (Mannheim) and reported a lifetime prevalence of about 12%.
Tjaden and Thoennes reported a lifetime prevalence (being stalked) of 8% in women and 2% in males (depending on how strict the definition) in the National violence against women survey.
Section 264 of the Criminal Code of Canada, titled "criminal harassment" addresses acts which are termed "stalking" in many other jurisdictions. The provisions of the section came into force in August 1993 with the intent of further strengthening laws protecting women. It is a hybrid offence, which may be punishable upon summary conviction or as an indictable offence, the latter of which may carry a prison term of up to ten years. Section 264 has withstood Charter challenges.
The Chief, Policing Services Program, for Statistics Canada has stated:
"... of the 10,756 incidents of criminal harassment reported to police in 2006, 1,429 of these involved more than one accused."
This is approximately one case in eight, and matches the percentage of cases reported by the U.S. Department of Justice, for the same period, which involve multiple stalkers, as opposed to the often presumed obsessed single stalker.
In 2000, Japan enacted a national law to combat this behaviour, after the Shiori Ino murder. Acts of stalking can be viewed as "interfering [with] the tranquility of others' lives and are prohibited under petty offence laws.
Following a series of high-profile incidents that came to public attention in the past years, a law was proposed in June 2008, and became effective in February 2009, making a criminal offence, punishable with imprisonment ranging from six months up to four years, any "continuative harassing, threatening or persecuting behaviour which: (1) causes a state of anxiety and fear in the victim(s), or; (2) ingenerates within the victim(s) a motivated fear for his/her own safety or for the safety of relatives, kins [sic], or others tied to the victim him/herself by an affective relationship, or; (3), forces the victim(s) to change his/her living habits". If the perpetrator of the offense is a subject tied to the victim by kinship or that is or has been in the past involved in a relationship with the victim (i.e. current or former/divorced/split husband/wife or fiancée), and/or if the victim is a pregnant woman or a minor, the sanction can be elevated up to six years of incarceration.
There is no offence which is described in law as "stalking". An attempt to create such an offence by the Stalking Bill 1996 failed. It was felt that the proposed offence failed to distinguish between reasonable and unreasonable conduct.
In England and Wales, "harassment" was criminalised by the enactment of the Protection from Harassment Act 1997, which came into force on June 16, 1997. It makes it a criminal offence, punishable by up to six months imprisonment, to pursue a course of conduct which amounts to harassment of another on two or more occasions. The court can also issue a restraining order, which carries a maximum punishment of five years imprisonment if breached. In England and Wales, liability may arise in the event that the victim suffers either mental or physical harm as a result of being stalked (see R. v. Constanza).
Already before the enactment of the Act, the Malicious Communications Act 1988 and the Telecommunications Act 1984 (now the Communications Act 2003) criminalised indecent, offensive or threatening phone calls and the sending of an indecent, offensive or threatening letter, electronic communication or other article to another person.
In Scotland, provision is made under the Protection from Harassment Act against stalking. It is not a criminal offence, however, but falls under the law of delict. Victims of stalking may sue for interdict against an alleged stalker, or a non-harassment order, breach of which is an offence.
The first state to criminalize stalking in the United States was California in 1990 due to several high profile stalking cases in California, including the 1982 attempted murder of actress Theresa Saldana, the 1988 massacre by Richard Farley, the 1989 murder of actress Rebecca Schaeffer, and five Orange County stalking murders also in 1989. The first anti-stalking law in the United States, California Penal Code Section 646.9, was developed and proposed by Municipal Court Judge John Watson of Orange County. Watson with U.S. Congressman Ed Royce introduced the law in 1990. Also in 1990, the Los Angeles Police Department (LAPD) began the United States' first Threat Management Unit, founded by LAPD Captain Robert Martin.
Within three years thereafter, every state in the United States followed suit to create the crime of stalking, under different names such as criminal harassment or criminal menace. The Driver's Privacy Protection Act (DPPA) was enacted in 1994 in response to numerous cases of a driver's information being abused for criminal activity, examples such as the Saldana and Schaeffer stalking cases. The DPPA prohibits states from disclosing a driver's personal information without consent by State Department of Motor Vehicles (DMV). The National Defense Authorization Act for Fiscal Year 2006 made stalking punishable under the Uniform Code of Military Justice (UCMJ). The law took effect on 1 October 2007. This law brings the UCMJ in line with federal laws against stalking. Laws against stalking in different jurisdictions vary, and so do the definitions. Some make the act illegal as it stands, while others do only if the stalking becomes threatening or endangers the receiving end. Many states in the US also recognize stalking as grounds for issuance of a civil restraining order. Since this requires a lower burden of proof than a criminal charge, laws recognizing non-criminal allegations of stalking suffer the same risk of abuse seen with false allegations of domestic violence.
According to the Department of Justice survey, a vast majority of stalking cases are not even taken to criminal court. Prosecutors often find it difficult to prove stalking beyond a reasonable doubt compared to other crimes.