When studying the population of individuals that commit stalking acts/crime, how often are these individuals considered, or diagnosed with Narcissistic Personality Disorder?
this is a tough question! The literature in this field is very unsatisfying unfortunately.
maybe, the following is relevant for you?!
1. The role of psychopathology in stalking by adult strangers and acquaintances by Troy E McEwan and Susanne Strand; published in Aust N Z J Psychiatry 2013; "The other type of psychopathology that is potentially relevant to stalking is personality disorder, which is present in 30–50% of stalkers in clinical/forensic samples (Mullenet al., 1999; Rosenfeld and Harmon, 2002). Cluster B disorders of the narcissistic, antisocial and borderline types are most common (Meloy, 2007; Sansone and Sansone, 2010). The best specific prevalence estimates come from studies of mental health professional victims, who report rates ranging from 35% to 45% in perpetrators (Galeazzi et al., 2005; Purcell et al., 2005; McIvor et al., 2008). Studies using diagnostic interview designs have not reported personality disorder by relationship type (Mullen et al., 1999; Rosenfeld and Harmon, 2002; McEwan et al., 2009), so any differences in personality pathology among ex-intimate, stranger and acquaintance stalkers remain unexamined. "
In this paper you also find an interesting table with regard to your question.
2. Personality disorder and criminal behaviour: what is the nature of the relationship? by Sophie Davisona and Aleksandar Jancaa; published in Curr Opin Psychiatry in 2012.
Mh, I don't know the above mentioned studies. But one should really be aware of the problem that personality disorders are rarely assessed properly in clinical practice. If there is no proper assessment with a blinded interview at intake I have a lot of doubt in the validity of the assessment because health care professionals may see the problem of stalking as a symptom of a personality disorder. There may be one, but I think if you ask the practitioners who are involved in the treatment of those persons, the question cannot be answered properly.