Sex differences in handedness and language lateralization

This chapter provides a quantitative review of sex differences in handedness, asymmetry of the planum temporale (PT), and cerebral lateralization of language. We have reviewed two reflections of language lateralization: asymmetric performance on dichotic listening tests (right-ear advantage) and asymmetry of functional language activation as measured with functional imaging techniques. Meta-analysis of studies that assessed handedness in males and females yielded a higher prevalence of left-handedness in males, with a mean weighted odds ratio of 1.29 (p < 0.000). Studies on anatomical asymmetry and functional language lateralization were less abundant and generally assessed much smaller samples than studies on handedness. Meta-analysis of studies on PT asymmetry yielded no sex difference in asymmetry (Hedge’s g = −0.05, p = 0.47). Results of the meta-analysis on dichotic listening studies were consistent with the findings for PT asymmetry; no sex difference in lateralization (Hedge’s g=0.04, p = 0.33). When the studies were subdivided according to the paradigm they applied, meta-analysis of studies that used the consonant-vowel task yielded significant sex differences favoring males. Meta-analyses of studies that applied other dichotic listening tasks yielded no sex difference. The subdivision into studies applying a certain paradigm largely overlapped with the subdivision into studies that did or did not focus on sex differences as theirmain topic. Analysis of an unpublished database that also used a consonant-vowel task and included three times as many subjects as our sub-analysis found no sex difference. It appears, therefore, more likely that the significant sex effect observed in a sub-analysis is caused by publication bias, than that the applied paradigm produces a sex difference in language lateralization. Consistent with the meta-analyses on PT asymmetry and on dichotic listening studies, analysis of functional imaging studies yielded no significant sex difference (Hedge’s g = 0.01, p = 0.73) in language lateralization. Sub-analyses of studies that applied word generation tasks, semantic decision tasks, or listening tasks all yielded no sex difference. In conclusion, males are more frequently left-handed than females, but there is no significant difference in cerebral asymmetry or in language lateralization between the sexes. Several factors may cause the increased left-handedness in males in the absence of decreased asymmetry or language lateralization. Higher antenatal testosterone levels appear to increase the chance for left-handedness. Alternatively, genetic factors could also lead to increased left-handedness in males. Left-handed mothers are more likely to have left-handed offspring than left-handed fathers. This suggests that sex-linked transmission, for example through an imprinted gene or a locus on the X-chromosome, may play a role in the genetics of handedness. Finally, sex differences in social pressure to use the right hand for writing and eating may explain the increased left-handedness in males. Females more frequently report to have switched in hand preference than males. Social factors would offer a valid explanation for the sex difference in handedness in the absence of associated sex differences in asymmetry and lateralization.