Psychopathology was rated from answers to detailed questions about symptoms and treatment. Psychopathology was divided into four categories: None, non-psychotic affec- tive conditions (overwhelmingly depression and anxiety disorders), schizophrenia (as evi- denced by a diagnosis of schizophrenia and treatment with anti-psychotics), and bipolar disorder (as evidenced by a diagnosis of mania and treatment with lithium). To maximise reliability, participants were assigned to psychopathology groups only if they reported and specified having receive a medical treatment (drugs or psychotherapy) for that condition, not if they report being troubled by the symptom but having had no treatment for it.
For poetry and visual art, participants were assigned to four groups on the basis of their responses: non-participant, hobbyist, serious, and professional. Non-participants groups were not active producers in the domain (though they could be keen readers of poetry or viewers of art). Hobbyists rated their production Ôas a hobby,Õ while the serious group described themselves as Ôseriously involvedÕ in the domain. Finally, the professional group made all or part of their living from the domain. For mathematicians, a simpler division was made into those who were active mathematical researchers and those who were not.
Note that membership of the different psychopathology and creative groups is indepen- dent, so one individual could simultaneously be in the non-mathematician group for math- ematics, the hobby group for poetry, the professional group for visual art, and the affective group for psychopathology. This means that, for example, the non-poet group contained the visual artists among others. This system of analysis was favoured for several reasons. First, it is statistically conservative, in that demonstrating an increase in schizotypy in poets is actually made harder by having groups of artists and psychiatric patients amongst the non-poet comparison group. Second, excluding all the hobby-level practitioners of the arts would lead to a large reduction in sample size. Third, the clinical and professionally creative groups formed a small proportion of the total sample.