A theory of the cause of mental retardation that views human intelligence as a set of independent abilities organized in a complex system was presented. Mental retardation is conceived as a deficit in a few of the independent abilities having high centrality (i.e., processes most important in system functioning). Abilities affected and severity of impairment vary across individuals. Besides making specific predictions that allow rejection, the theory can be used to account for differences between Ellis's deficit theory and Zigler's developmental theory of mental retardation.