Handedness and Speech: A Critical Reappraisal of the Role of Genetic and Environmental Factors in the Cerebral Lateralization of Function

Functional predominance of the left cerebral hemisphere with regard to both handedness and speech has usually been assumed to be due to some underlying neural specialization that is predetermined and inborn. However, data from left-handed individuals and animal experiments, together with a consideration of the effects of natural selection on brain and behavior during hominid evolution, are incompatible with such an explanation. A critical reexamination of the relevant nonhuman and human evidence suggests that although the development of a cerebral lateralization for speech and handedness is dependent on both genetic and environmental factors, the specific role of inborn and postnatal influences is very different. This has significant implications for a fundamental revision of current theory and research orientation

The specificity of motor skill and manual asymmetry: A review of the evidence and its implications

Results of investigations on gaining control of limb movements are reviewed, and their contribution to understanding the development of manual asymmetries is discussed in relation to the discrimination and programming of appropriate neuromuscular resources. An examination of the relevant evidence on number and types of manual asymmetries recorded provides strong grounds for concluding that where asymmetries occur, they simply represent a further example of the well-documented activity-specific nature of motor skills and of the extremely lengthy periods of learning or experience needed for their acquisition and perfection. This specificity of motor skill and manual asymmetry also readily accounts for most of the discrepancy usually reported between assessments of hand preference and performance differences between hands, because these alternative measures of handedness have rarely employed the same range or variety of tasks

Early Infant Motor Asymmetries and Handedness: A Critical Evaluation of the Evidence

Evidence relating to early infant motor asymmetries is critically evaluated with respect to (a) hand and arm movements, (b) head turning and the rooting reflex, (c) postural orientation, and (d) parental holding and carrying practices. The evidence suggests that where asymmetries occur, they may be temporary developments that reflect the cumulative pre- and/or postnatal experiences of the infant up to that time. Claims of early infant and other asymmetries predicting adult handedness are considered doubtful in view of the handedness criteria employed. Spontaneous fluctuations in both the absolute and relative activity of the right and left hands characteristic of infancy may also account for some of the otherwise incompatible findings reported previously. It is suggested that there is currently insufficient behavioral evidence to conclude that the cerebral lateralization of motor functions is based on a hemispheric specialization that is fixed at birth and is unchanging thereafter. Some possible lines for future research are indicated