Methods are developed for the resolution of cultural and biological inheritance based on qualitative data on pairs of relatives. Analysis of data on schizophrenia concluded in favor of significant cultural inheritance, assortative mating and familial environment unique to twins, in addition to biological inheritance. Genetic and cultural heritabilities were estimated as 0.707 +/- 0.077 and 0.203 +/- 0.120, respectively. Familial environmental component unique to twins was estimated as 0.090 +/- 0.040.