Sex differences in handedness, asymmetry of the planum temporale and functional language lateralization.

Many studies have investigated sex differences in language lateralization. Despite the large number of investigations, controversy about the presence of sex differences in lateralization remains. This study aims to provide a complete overview of sex differences in several reflections of language lateralization: handedness, asymmetry of the Planum Temporale (PT) and functional lateralization of language, measured by asymmetric performance on dichotic listening tests (Right Ear Advantage) and asymmetry of language activation as measured with functional imaging techniques. Meta-analysis of studies that assessed handedness in males and females yielded more left-handedness in males (mean weighted odds ratio: 1.25, p<0.001). Meta-analysis of studies on PT asymmetry yielded no sex difference (Hedges g=-0.11, p=0.68). Results of the meta-analysis on dichotic listening studies also retrieved no sex difference in lateralization (Hedges g=0.09, p=0.18). When the studies were subdivided according to the paradigm they applied, studies that used the consonant-vowel task yielded a sex difference favouring males, while studies that applied other paradigms yielded no sex difference. The subdivision into applied paradigm largely overlapped with the subdivision into studies that did or did not focus on sex differences as their main topic. The observed sex effect may therefore be caused by publication bias. Meta-analysis of functional imaging studies yielded no sex difference (Hedges g=0.01, p=0.73) in language lateralization. Sub-analyses of studies that applied different paradigms all yielded no sex difference. In conclusion, males are more frequently non-right handed than females, but there is no sex difference in asymmetries of the Planum Temporale, dichotic listening or functional imaging findings during language tasks.