Language and reading impairments are associated with increased prevalence of non-right-handedness

Worldwide, the majority of people prefer using the right hand for most motor tasks. Because of the link between handedness and language hemispheric dominance, handedness has been studied for association with language-related disorders. No clear pattern has emerged from these studies, and inconsistencies have been attributed to small sample sizes, publication bias, and heterogeneous criteria for the definition of handedness and disorders. Here, we assessed the frequency of non-right handedness (NRH) in 10 distinct cohorts not analysed before in this context. We identified N = 2,528 cases with reading and/or language impairment and N = 3,050 unique controls on the basis of a priori defined criteria. Overall, NRH was more frequent and more variable in the cases (8-24%) than in the controls (8-16%). Meta-analysis in the eight cohorts that met the inclusion criteria showed an increase of NRH in individuals with language/reading impairment compared to controls (OR = 1.21, CI = 1.03 - 1.44, p = 0.025). No moderator effects were detected for type of cohort (epidemiological versus clinical) and type of impairment (language versus reading). Our results support an association between NRH and reading and language impairments which could result from shared biological pathways underlying brain lateralization, handedness, and cognitive functions