Schizoid personality disorder has a complicated past. It was originally conceptualized as the personality type associated with schizophrenia. However, the Diagnostic and Statistical Manual of Mental Disorders states that schizoid personality disorder should not be diagnosed if the pattern of behavior occurs exclusively during the course of schizophrenia. But this does not mean that the two cannot co-occur. Schizoid personality disorder most often co-occurs with schizotypal, paranoid, and avoidant personality disorders. At the turn of the twentieth century, schizoid personality disorder was described as a trait-like variant of schizophrenia. Others described it as "shut-in personality," "schizoidie," and even as "autistic personality." The term schizoid was also used in broader terms to identify socially withdrawn patients who had great difficulty with intimacy and peculiar behavior now associated with schizotypal personality disorder. .
However schizoid personality disorder can be differentiated from other disorders. Schizoid personality disorder is different from schizotypal personality disorder in that schizotypal personality disorder lacks the cognitive and perceptual distortions that are associated with schizoid personality disorder. It can be distinguished from paranoid personality disorder by the lack of suspiciousness and paranoid ideation. The social isolation of schizoid personalities can be distinguished from that of avoidant personalities by a lack of fear of being embarrassed or found inadequate and an excessive anticipation of rejection. It can be distinguished from autistic disorder but not easily. Milder forms of autistic disorder are very similar to schizoid personality disorder, so differentiation is very difficult. This is also true for asperger's disorder. However, they can be differentiated. Their symptoms both exhibit more severely impaired social interaction and stereotyped behaviors and interests.