The hypothesis that there is a genetic balanced polymorphism with heterozygote advantage (BP + HA) for the RS locus cannot be examined directly because the relevant alleles are unknown. What can be asked at present is whether there are associations between laterality and ability that are consistent with a BP + HA. There is a large literature on laterality and abilities, but there are few undisputed findings. Previous research has looked for statistically significant differences between left-versus right-handers, treating handedness as a type variable. The RS model and BP + HA hypothesis expect relationships to be more subtle because handedness is not a type, and the relevant genotypes are not “for” handedness but “for” different patterns of cerebral specialisation. The RS − −, RS + − and RS + + genotypes imply that a bias to left CD is absent, moderate and strong respectively. Because of the substantial overlap between genotypes (see Figure 7.1) contrasts for handedness are expected to be small. However, the model makes predictions about the different types of ability that might be at risk versus advantage in the two homozygotes. The overall advantage for heterozygotes might depend, therefore, on minimising both types of risk and achieving overall balance for cognitive and other abilities. The present chapter looks for evidence of heterozygote advantage, but it is useful to begin with a summary of the assumptions involved in the BP + HA hypothesis, so that the strategies for research described in this and the next two chapters can be seen as a whole.