The idea that the right shift of the handedness distribution depends on a single gene (RS +) whose main function is to induce the left cerebral hemisphere to serve speech was shown in chapter 5 to be consistent with findings for handedness in families. The genetic calculations did not distinguish the sex of parents or children, nor RS + − and RS + + genotypes for extent of shift, for reasons explained. However, certain findings in chapters 5 and 6 suggested that these distinctions should be made. The RS + gene is hypothesised to be dominant for cerebral asymmetry in the sense that one copy of the gene is sufficient to promote left CD in the course of normal growth. However, the gene could be additive in the sense that the mechanisms promoting cerebral asymmetry give greater displacement to the right for handedness, and perhaps larger asymmetries of cerebral anatomy, when there are two copies than only one. Further, there may be differences between the sexes and between twins and the singleborn in gene expression. That is, different levels of shift.