Chapter 14: Individual Differences: Gender and Handedness

Regarding gender differences, historical claims that males possess superior spatial abilities while females excel in verbal tasks have prompted neuropsychologists to propose links between these cognitive patterns and lateralization of cerebral function. Early models such as the Buffery and Gray hypothesis suggested males showed reduced lateralization, though contemporary research often inverts this claim, proposing instead that female brains demonstrate greater bilateral organization. Clinical evidence from brain-injured populations reveals that males typically display clear-cut deficits corresponding to lesion location, with left-hemisphere damage affecting verbal abilities and right-hemisphere damage affecting performance abilities, whereas females often lack these predictable patterns. However, experimental studies using divided visual field techniques have produced inconsistent and contradictory findings. Meta-analytic reviews indicate that population-level differences exist but remain extremely modest, accounting for merely one to two percent of observed variance and therefore possessing minimal predictive utility for individual cases. Some researchers attribute apparent differences to cognitive strategy preferences rather than underlying neurobiological variation. Additionally, research on sexual orientation suggests homosexual individuals, particularly women, show elevated rates of non-right-handedness, implying possible connections to differential neurodevelopmental organization. The chapter then shifts focus to handedness, present in approximately eight to twelve percent of the population as left-handedness. Marian Annett's influential right shift theory proposes that a single genetic factor predisposes individuals toward right-hand preference, while its absence leaves handedness determination to random processes. Several competing models attempt to explain left-handedness through pathological mechanisms such as birth trauma or stress, though contemporary evidence suggests such explanations account for only minimal cases. The contralateral rule asserting that speech dominance opposes hand preference lacks substantial scientific support. Evidence from the Wada technique and aphasia studies demonstrates that approximately seventy percent of left-handers show left-hemisphere speech dominance, with remaining individuals exhibiting substantial bilateral representation. The chapter concludes that gender differences in cerebral organization, while likely genuine, are so subtle as to carry little practical significance, whereas handedness consistently correlates with reduced cerebral lateralization and more diffuse bilateral functional organization.