Direct and indirect behavioral effects of genetic disorders of mental retardation were identified. Three models of direct effects were examined: the no-specific effect model (all genetic disorders lead to identical behavioral outcomes), the totally specific model (each genetic disorder leads to unique outcomes), and the partially specific model (two or more genetic disorders lead to outcomes not shared by others with mental retardation). Although several cases of totally specific outcomes have been identified, partially specific effects most often occur. Persons in the surrounding environment are indirectly affected by behavioral propensities of different genetic disorders. Direct and indirect effects are also related to biological and developmental theory.