Implications of the Right Shift Theory of Handedness for Individual Differences in Hemisphere Specialisation

Abstract

Two main approaches to problems of individual differences in hemisphere specialisation are to be found in the literature. The first is avoidance: subjects are restricted to fully right-handed males, with no known left-handed relatives. It is assumed that such subjects are likely to be homogeneous for the typical pattern of cerebral specialisation. The second approach is to compare subjects for personal hand preference, or for the presence of left-handed relatives, usually taking care to treat the sexes separately, in the expectation that these variables will be associated with differing patterns of cerebral specialisation. The right shift (RS) theory of handedness (Annett, 1972) suggests that the homogeneity of subjects in the first approach, and the discriminating power of variables in the second approach, are overestimated. Some of the challenges of the RS theory were evident from its initial formulation, and others have been discovered in subsequent explorations of it’s implications. A brief review of the development of the theory was given by Annett (1981) and a full review by Annett (1985). This paper summarises implications of the theory for individual differences, giving first an overview, and then a selective review of evidence for the main assumptions.