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General Characteristics of Schizophrenia

Three traits were observed more frequently among the biological relatives of schizophrenic probands: suspiciousness, flat affect, and withdrawn behavior. Psychotic-like features (e.g., cognitive and perceptual distortions) were not observed. Subsequent twin studies have replicated these findings. In a series of studies conducted with the Norwegian Twin Registry, Torgersen found a higher frequency of SPD among MZ co-twins of probands with SPD compared with DZ co-twins (33% vs. 4%), as well as increased rates of SPD in the MZ co-twins of schizophrenics compared with DZ co-twins (20% vs. 14.8%). Personal interviews showed that feelings of isolation, social anxiety, and inadequacy were common among the co-twins of schizophrenic probands and that inappropriate affect and odd speech and behavior also were common among f irst-degree relatives. Psychotic-like phenomena were absent once again. Torgersen et al. concluded that schizotypal features were genetically transmitted but that the only heritable aspects of SPD were the affect-constricted and eccentric features.

Further evidence that the negative symptom criteria of SPD may be heritable independently of the positive symptom criteria comes from Kendler, Ochs, Gorman, and Hewitt’s  factor analysis of schizotypal symptoms in normal MZ twins. The authors’ factor analyses revealed two independent dimensions of schizotypy, one related to the social withdrawal and deficit symptoms, the other related to the positive, or psychoticlike, symptoms. Thus findings from behavior genetic studies of SPD seem to consistently point to the negative symptom criteria of SPD as those responsible for its familial association with, and genetic relatedness to, schizophrenia. Negative symptoms may be a more direct reflection, or true manifestation, of the underlying schizophrenic genotype—an interpretation consistent with Bleuler’s early descriptions. Evidence that supports the inclusion of DSMIV disorders other than SPD in the schizophrenia spectrum is less consistent. Elevated rates of paranoid personality disorder (PPD) were found in the biological relatives of schizophrenic adoptees, compared to controls in the Copenhagen sample of the Danish Adoption Study. In contrast, Lowing et al. did not find a higher frequency of PPD in the adopted-away offspring of schizophrenic, compared to control parents, but did find increased rates of schizoid personality disorder (10% vs. 3%). The relationships between these personality disorders and schizophrenia may be an artifact, however. Both PPD and schizoid personality disorder are defined by a subset of SPD criteria; thus diagnoses of either may be coextensive with a diagnosis of SPD. The literature on psychotic disorders is also inconsistent. Several family studies have found higher rates of schizoaffective disorder among the relatives of schizophrenic than among control probands. Some evidence for a familial relationship between nonaffective psychoses and schizophrenia also exists, and studies found increased rates of schizophreniform disorder, delusional disorder, and/or atypical psychosis in the families of schizophrenic versus control probands. Kendler and Gardner  computed a common OR estimate of 4.0 for ‘‘other non-affective psychoses’’ across three independent family studies. These included the DSM-III/DSM-III-R categories of schizophreniform disorder, schizoaffective disorder, delusional disorder, and atypical psychosis (psychosis not otherwise specified). In sum, the boundaries of the schizophrenia spectrum extend from schizophrenia-like personality traits such as poor social functioning, odd thoughts and behavior, and suspiciousness at one end, to the full-blown disorder of classical schizophrenia at the other. Included within these boundaries are the DSM-IV disorders of SPD, possibly PPD, and probably several other nonschizophrenic psychotic disorders.

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