In contrast to the controversy surrounding the classification of schizoaffective disorder, family, twin, and adoption studies clearly support the view that schizotypal personality disorder is best classified in the schizophrenia spectrum. (29> Nevertheless, it is a complex and chronic disorder that in all likelihood, is also heterogeneous. Kendler(30> pointed out that this heterogeneity was at least partly related to the two primary methods used to study the disorder. One of these involves the 'clinical method', which identifies patients with mild forms of schizophrenic or psychotic-like symptoms. Thaker et al. (31> for example, reported that clinical schizotypal subjects were characterized by relatively high levels of positive psychiatric symptoms (e.g. magical thinking and perceptual distortions).

In contrast, the 'family research method' identifies relatives of patients with schizophrenia who have subtle, schizophrenia-like symptoms. Features associated more with familial than with clinical schizotypal personality disorder include a predominance of negative symptoms (e.g. social withdrawal and impairment, and higher levels of anxiety and poor rapport), cognitive impairments (e.g. impaired language comprehension, eye-tracking, and attentional dysfunctions), and elevated rates of schizophrenia and related disorders in family members. (29> Thaker et al.(31> reported that familial and clinical schizotypal personality disorders were similar on measures of physical or social anhedonia. Also, some neuropsychological deficits are associated with both groups. (3,33>

The concept of familial schizotypal disorder is particularly important because it may share a common genetic basis with schizophrenia. Meehl (34> first proposed the term 'schizotaxia' to describe the genetic vulnerability to schizophrenia, and suggested that individuals with schizotaxia would eventually develop either schizotypal personality disorder or schizophrenia, depending on the protection or liability afforded by environmental circumstances. As the concept evolved, Meehl (35) reformulated it to allow for the possibility that some people with schizotaxia would develop neither schizophrenia nor schizotypal personality disorder. In fact, evidence now shows that the clinical symptoms observed in many non-psychotic, first-degree relatives of people with schizophrenia are similar to those observed in familial schizotypal personality disorder. (29> Psychiatric features in such relatives frequently include an aggregation of negative symptoms that are qualitatively similar to, but milder than, those often cited in schizophrenia. (36> Positive symptoms, however, are usually less evident in these relatives than they are in schizophrenia or in schizotypal personality disorder. Neuropsychological impairments in biological relatives of people with schizophrenia are also qualitatively similar to, but milder than, those seen in people with schizophrenia. (29> In particular, these neuropsychological deficits frequently include problems in working memory/attention, long-term verbal memory, and concept formation/abstraction.

Faraone et al. recently suggested a reformulation of Meehl's concept of schizotaxia that focuses on these features of negative symptoms and neuropsychological deficits/29' Unlike schizotypal personality disorder, which occurs in less than 10 per cent of the adult relatives of patients diagnosed with schizophrenia, the basic symptoms of schizotaxia occur in 20 to 50 per cent of adult relatives,(3 38> suggesting further that the genetic liability to schizophrenia does not lead inevitably to schizophrenia, schizotypal personality disorder, or schizoid personality disorder.

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