The zygosity of the twin pairs is then investigated and the prevalence of ‘schizophrenia’ in identical and same-sex fraternal co-twins is examined. The question of interest is how this disparate set of results, from studies with, as will be seen, considerable methodological and conceptual problems, has come to be presented as part of the “incontrovertible evidence” that schizophrenia has a genetic basis. Subjects initially selected for study are usually called probands or index cases. It is not difficult to see how such methods could lead to innacurate conclusions. Researchers’ judgements as to similarity of appearance may have been influenced by personal biases, not the least of which was their opinion about diagnoses and knowledge of whether one or both twins had been hospitalised. Personal observation was often supplemented or replaced by questioning twins’ relatives about confusion of identity, similarity of appearance, and so on.