Individuals with Schizoid Personality Disorder are highly detached from nearly all social relationships and have a restricted range of emotions in interpersonal interactions. A schizoid person is the quintessential loner, who appears aloof, cold, and remote to others. Even when pressed into relationships, they do not appear to enjoy them, including sexual interactions. Individuals with Schizotypal Personality Disorder share two of the same criteria as the Schizoid Personality Disorder: (1) constricted affect and (2) a lack of close confidants. Like schizoid persons, schizotypal persons have serious social and interpersonal difficulties. Unlike schizoid persons, schizotypal persons have in addition eccentricities in their thinking and reasoning, such that they find highly personal meanings in meaningless events (ideas of reference) and have odd beliefs and speech or magical thinking. Like the Schizoid and Schizotypal Personality Disorders, individuals with Avoidant Personality Disorder are also socially inhibited. Avoidant Personality Disorder traits also include negative evaluations of self and great restraint in social and interpersonal relationships, which are both longed for and feared.
Some individuals who are either forced into seclusion (by the death of their family) or who socially or interpersonally withdraw for other reasons may function at least semi-normally by themselves; sometimes they even do original work on their own. One factor for that success might be that their seclusion may foster conditions that allow or generate creativity and originality (e.g., fewer social or emotional distractions).
Another factor is that diseases (particularly fatal ones) may have been transmitted more readily with increasing social interaction. A schizoid, schizotypal, or avoidant individual in the ancestral environment, who reluctantly participated in social or sexual unions, might have been subsequently much less susceptible to diseases that were socially transmitted. It might also be possible that the restricted emotional expressions in these individuals (having "poker faces") would make them less vulnerable to manipulation and being taken advantage of when engaging in reciprocal altruistic acts.
In summary, evolutionary psychologists believe that personality disorders persist because of frequency-dependent selection. Behavior traits that do not predominate and have rather low prevalence rates survive because individuals with these traits found adaptive niches in the ancestral environment.
Evolutionary psychologists have not yet directly addressed the issue of personality disorders and aging, but numerous general evolutionary theories of aging exist. Kirkwood (2000) suggested that aging and senescence were the price humans paid for their evolutionary success. Indeed, our somatic cells age while our genes have the potential to be immortal. Bouchard and Loehlin (2001) observed that evolutionary psychologists, although addressing the evolutionary adaptiveness of personality disorder traits in the ancestral environment, have not yet adequately studied personality disorders in the evolutionary perspective in regards to aging.
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