Birth characteristics and schizotypy: evidence of a potential "second hit".

Schizophrenia is associated with a modest increase in winter births as well as increased odds of being born in more densely populated and midrange latitude regions. It is unclear the degree to which these findings hold for individuals with schizotypy, defined in terms of the personality organization that is a potential precursor to schizophrenia-spectrum disorders. This issue is important for understanding whether birth factors contribute to general schizophrenia vulnerability or whether they reflect a secondary "hit" that increases the likelihood of psychosis onset in vulnerable individuals. The present project examined season of birth, birthplace population and birth location in a large group of young adults from the southeastern United States. Individuals with extreme schizotypy scores did not differ from those without schizotypy in season of birth, birthplace latitude or population. However, 60% of individuals within the schizotypy group who reported a diagnosis of schizophrenia or prior hospitalization were born during winter months; a dramatic difference from other individuals within the schizotypy group. We also found that individuals with negative/schizoid traits showed a birthplace population less than half that of other individuals with schizotypy. Season of birth appears to be a "second hit" that is related to expression of psychopathology onset in vulnerable individuals. This finding, and the unexpected inverse relationship between birthplace population and negative/schizoid traits, is discussed.