In an effort to understand, and therefore reduce or eliminate crime, studies are conducted that examine the many factors that influence or affect it. One of the influences [or possible influences] that affect crime [particularly violent crime]is mental disorder. Several studies have been carried out that investigate this link, which have uncovered much important information. These studies [as with most studies] however, are not without bias. Things such as definition of mental disorder or abnormality are often questionable, as are sample distributions. There are also individual factors, such as type of mental disorder and previous criminal records of subjects, which can influence the results. After taking these factors into account, the research results when examined, suggest, contrary to popular belief, that individuals with mental disorders are generally no more likely to commit crime than individuals without mental disorder.
It is a common assumption, that whenever a brutal, violent or senseless crime is committed, it is by someone who is mentally ill or sick. Blaming violent and senseless crime on mental disorders may be comforting but it is not necessarily accurate. A stereotype has developed of the 'insane mass murderer' largely due to the media, however, research evidence suggests that this stereotype is far from accurate. Much research on the link between crime [specifically to this discussion, violent crime] and mental disorder has been conducted, however there are different factors that need to be taken into account when examining this link. Definition of mental disorder is a major one, along with possible uneven sample distributions. Also, there have been changes in mental health and criminal justice policies that have increasingly made hospitalisation restricted to those who are more socially disruptive or dangerous. With these considerations in mind, research evidence can be then be examined.
As previously stated, there are many problems with the mental disorder definition. The major symptom of mental disorder is abnormal behaviour, however, there is no strict criteria that sufficiently defines abnormality. Behaviour could be labelled abnormal if it deviates from the statistical norm, however, as the American Psychiatric association  notes 'there are no sharp boundaries between normality and abnormality'. It can be concluded from this then, that there are no sharp boundaries between mental order and mental disorder. Behaviour could also be labelled abnormal if it deviates from the cultural norm. This is problematic because what society deems as 'normal' is subjective and constantly changing. For example, homosexuality was regarded as being a mental disorder until 1973. These are only two of the ways in which mental disorder is possibly identified but others, such as behaviour that 'is subjectively distressing, deviates from optimal social or psychological functioning or fails to meet some ideal of health' [Blackburn, 1993, p.247] are all subjective and are by no means exact. As such, these classifications have numerous exceptions. Another element of research on the mental disorder and crime link that is often questioned, is the sample distribution.
Some believe that many of the studies conducted in this area have an uneven sample distribution that introduces bias in the results. As with many studies conducted in the criminology field, there is the question: should only those convicted of crime [and in this case, those officially diagnosed with a mental disorder] be studied or, should only those who indulge in criminal activity but have not yet been detected by the criminal justice system [or those with undiagnosed mental disorders] be studied, or both? Furthermore, if it is decided that both should be studied, how can people who commit crime, but have not been convicted, or people with mental disorders who have not been diagnosed, be included? These are difficult questions that are up...