ABSTRACT
The research outlined so far began with the puzzle of relationships between handedness and cerebral speech laterality. Distributions of hand preference and hand skill led to the idea that the only thing that is special about human handedness, in comparison with other species, is that a universal chance distribution of asymmetry is displaced in humans in a dextral direction. This theory led to several useful and surprising discoveries. First, if the agent of displacement is a cerebral asymmetry that normally leads to left CD but is absent in some people, then relations between handedness and left versus right CD are predictable from the model. Second, if this agent is a single gene (RS +) with alternative alleles at the same locus (RS −) neutral for laterality, then findings for handedness in families are predictable. The model was strengthened by the fact that gene and genotype frequencies were deduced from the percentages of patients with typical versus atypical CD, not from the genetic data. The theory led to further productive findings, including differences in gene expression between the sexes and between twins and the singleborn, the predictability of eye dominance in families, and the predictability of right and left pairings between different asymmetries (as outlined in chapters 2–5). It cannot be doubted that the RS genetic model offers solutions to several problems. However, solutions to one set of problems often lead to the challenges of new problems. The gene and genotype frequencies deduced above pose a very profound puzzle. Why is the frequency of the RS + gene only 0.57, while 0.43 alleles remain neutral for laterality?
