鈥� This article provides an in-depth analysis of hallucinations in monozygous quadruplets concordant for schizophrenia. Since all four are genetically identical, deviations among them with regard to various aspects of hallucinations probably represent the effects that nongenetic factors can produce in hallucinating schizophrenics. Concordances probably represent the effects of genetic factors. We present here findings based on this research strategy, and a brief review of the literature on hallucinations.