It is notable that before any empirical literature existed, the belief that ‘schizophrenia’ or ‘dementia praecox’ had a genetic basis was already well established. Kraepelin (1899) claimed, without providing any supporting evidence, that ‘[a]n inherited predisposition to mental disturbances was apparent in approximately seventy per cent of those cases in which data could be evaluated’ (203). Some clues as to how he reached this conclusion were provided by Kraepelin’s comment that ‘[b]odily signs of degeneracy were often found – a small or deformed skull, childlike appearance, defective teeth, misshapen ears, strabismus, masses of warts on the chest, general feebleness and indications of an easily excited brain’ (203). Bleuler shared Kraepelin’s beliefs, and his lack of evidence:
[I]f an adherent of an ‘infectious theory’ of this disease should choose to say that there is no hereditary factor in schizophrenia but merely an infection from some common source . . . we would be unable to produce any proof to the contrary. Such skeptics could observe that in many cases, even after the most thorough study, no evidence of any hereditary anlage and no individual predisposition has ever been proven. And yet heredity does play its role in the etiology of schizophrenia . . .