The likelihoods of observing 25 four-generational families of schizophrenics comprising 1,333 individuals have been calculated on the basis of 12 different genetic models and one control ‘sporadic’ model. The control model gave a log10 likelihood, L, of –240.92. Five of the genetic models were definitely excluded as incompatible with certain pedigrees. The three models with the highest likelihoods were: one locus, the heterozygote having a 10% probability of being classified schizophrenic (L: –220.05); two interacting loci (L: –219.46), and four polygenes (L: