OBJECTIVE: The aim of the study was to investigate the type and nature of personality disorders among biological relatives of schizophrenic probands. METHOD: A total of 176 nonschizophrenic co-twins and other first-degree relatives of schizophrenic probands were compared to 101 co-twins and first-degree relatives of probands with major depression. RESULTS: Schizotypal personality disorders were more common and histrionic personality disorders less common among the biological relatives of schizophrenic probands than among relatives of probands with major depression. A further exploration of the schizotypal criteria revealed that the so-called "negative" criteria such as odd speech, inappropriate affect, and odd behavior, as well as excessive social anxiety, were significantly more common among the relatives of schizophrenic probands. The latter criterion seems particularly important. The so-called "positive" schizotypal criteria were partly, although not statistically significantly, more common among the relatives of probands with major depression. There were only minor differences in frequencies of the negative criteria between monozygotic co-twins, dizygotic co-twins, and other first-degree relatives of schizophrenic probands. CONCLUSIONS: The present study suggests that DSM-III-R schizotypal disorder is defined by a set of criteria that partly describe a "true" schizophrenia-related personality disorder and partly features that are not specific for relatives of schizophrenic probands. Furthermore, the genetic relationship between schizophrenia and "true" schizotypal personality disorder seems weak. Excessive social anxiety may be a marker of a possible genetic link between the disorders.Abstract Teaser