Schizoid personality disorder is characterized by the sufferer's need for social isolation and solitary lifestyle. In contrast to schizotypal personality disorder, the sufferers social isolation is not a result of disability in social situations, but simply a lack of interest in a social life. This is a rare disorder, with less than 1% of the population believed to suffer from it.
The cause of this disorder is unknown, though some studies suggest genetics and family situations may affect the risk factor. Lack of social stimulation as a child often increases the risk of this disorder.
A person who is schizoid will avoid close relationships and may appear cold and unemotional to others. They prefer solitude and avoid sex, not because they do not enjoy the experience, but because they resent the emotional closeness they would have to develop with their partner. They often prefer masturbation or abstinence to the emotionally involved sexual experience. A schizoid person may lack the pleasure and fun most people get from leisure activities due to their reduced emotional spectrum. Their lack of interest in others means they are often indifferent to praise or criticism from others.
PREVALENCE AND SUSCEPTIBILITY
It is not known what the actual cause of this disorder is, but those who were not socially stimulated as children by their parents or have a family history of schizophrenia may be more at risk. Males are diagnosed with this disorder slightly more than females.
Medication is not used as a treatment of this disorder and psychotherapy is usually the sole treatment.Therapy often involves a build up of trust between patient and therapist. Self help groups are also part of treatment. Group therapy is a way of easing the patient into a social setting and introducing them to others with personality disorders that are similar.
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 - http://psychologytoday.com/conditions/schizoid.html
 - http://www.minddisorders.com/Py-Z/Schizoid-personality-disorder.html