Modern theories of genetic influence on lateralization suggest that random or fluctuating asymmetry has an important role. Theories of directional asymmetry or chance can be distinguished from the right shift (RS) theory that accidental asymmetries are universal for bilaterally symmetrical organisms. The right shift (RS) theory developed through a series of stages of empirical research and theoretical analysis. Briefly, the theory suggests that degrees of hand preference map onto a non-genetic normal distribution of asymmetry for hand skill, a continuum of right minus left (R - L) skill. The RS model explains relations between handedness and cerebral speech laterality. The key argument, for most skeptics of genetic influence on handedness, is the discordance of monozygotic (MZ) pairs. An early attempt to study the inheritance of brain asymmetries used a dichotic listening test in 49 families.