Dual functional brain asymmetry refers to the notion that in most individuals the left cerebral hemisphere is specialized for language functions, whereas the right cerebral hemisphere is more important than the left for the perception, construction, and recall of stimuli that are difficult to verbalize. In the last twenty years there have been scattered reports of sex differences in degree of hemispheric specialization. This review provides a critical framework within which two related topics are discussed: Do meaningful sex differences in verbal or spatial cerebral lateralization exist? and, if so, Is the brain of one sex more symmetrically organized than the other? Data gathered on right-handed adults are examined from clinical studies of patients with unilateral brain lesions; from dichotic listening, tachistoscopic, and sensorimotor studies of functional asymmetries in non-brain-damaged subjects; from anatomical and electrophysiological investigations, as well as from the developmental literature. Retrospective and descriptive findings predominate over prospective and experimental methodologies. Nevertheless, there is an impressive accummulation of evidence suggesting that the male brain may be more asymmetrically organized than the female brain, both for verbal and nonverbal functions. These trends are rarely found in childhood but are often significant in the mature organism.

Sex differences in human brain asymmetry: a critical survey

Life span studies have shown that the population percentage of left-handers diminishes steadily, so that they are drastically underrepresented in the oldest age groups. Data are reviewed that indicate that this population trend is due to the reduced longevity of left-handers. Some of the elevated risk for sinistrals is apparently due to environmental factors that elevate their accident susceptibility. Further evidence suggests that left-handedness may be a marker for birth stress related neuropathy, developmental delays and irregularities, and deficiencies in the immune system due to the intrauterine hormonal environment. Some statistical and physiological factors that may cause left-handedness to be selectively associated with earlier mortality are also presented.

Left-handedness: a marker for decreased survival fitness.

Lateral eye movements and hemispheric asymmetry: a critical review.

Individual differences in cognitive abilities and brain organization, Part I: Sex and handedness differences in ability.

Since prehistoric times, left-handed individuals have been ubiquitous in human populations, exhibiting geographical frequency variations. Evolutionary explanations have been proposed for the persistence of the handedness polymorphism. Left-handedness could be favoured by negative frequency-dependent selection. Data have suggested that left-handedness, as the rare hand preference, could represent an important strategic advantage in fighting interactions. However, the fact that left-handedness occurs at a low frequency indicates that some evolutionary costs could be associated with left-handedness. Overall, the evolutionary dynamics of this polymorphism are not fully understood. Here, we review the abundant literature available regarding the possible mechanisms and consequences of left-handedness. We point out that hand preference is heritable, and report how hand preference is influenced by genetic, hormonal, developmental and cultural factors. We review the available information on potential fitness costs and benefits acting as selective forces on the proportion of left-handers. Thus, evolutionary perspectives on the persistence of this polymorphism in humans are gathered for the first time, highlighting the necessity for an assessment of fitness differences between right- and left-handers.

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Why are some people left-handed? An evolutionary perspective

Since Day ( 3 ) first described lateral eye movement, i.e., the lateral movement of the eyes in response to reflective questions, investigators (1, 2, 9 ) have reported differences in the cognitive abilities of consistent right and left movers which were in general agreement with the differences in ability associated with the functions of the left and right cerebral hemispheres (5, 7, 8 ) . Assuming the validity of the aforementioned relationship, it is surprising to note that both Bakan ( 1 ) and Ecaugh (4 ) reported that direction of lateral eye movement and handedness were unrelated. Further, apparently no one has reported a relationship between direction of lateral eye movemenr and sex. Since both handedness ( 5 ) and sex ( 6 ) have been associated with patterns of abilities that seem to be related to differential functioning of the cerebral hemispheres, it was thought that the relationships between direction of la:eral eye movement and both handedness and sex should be investigated. The subjects were 23 female and 15 male undergraduates, of which 21 considered themselves right-handed and 17 left-handed. Using Bakan's ( 1 ) criteria, 16 of these subjects showed strong right lateral eye movement and 22 showed strong left movement. Significant associations were found between direction of movemenr and both handedness and sex. Specifically 16 of 22 left-movers were right-handed and 1 1 of 16 right-movers were left-handed ( x a = 6.45, p < .025, Phi = .412). In addition, 10 of 16 right-movers were male and 17 of 22 left-movers were female ( X 2 = 6.13, p < .025, Phi = ,402). These findings should be interpreted as identifying variables which require attention in future research on lateral eye movements. If these data are supported, then perhaps previous relationships reported between lateral eye movements and patterns of ability should be reevaluated since these results may be confounded by handedness and/or sex.

Robert Beveridge

Papers

Lateral Eye Movement, Handedness and Sex:

Handedness and intelligence.