Non-right handedness is associated with language and reading impairments

Worldwide, the majority of people prefer using the right hand for most motor tasks, including writing. 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 = 2,499 cases with reading and/or language impairment and N = 4,428 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 = 1.06 - 1.37, p = 0.009). No moderator effects were detected for type of cohort (epidemiological versus clinical) and type of impairment (language versus reading). Our results support a genuine but modest association between NRH and reading and language impairments, suggesting shared biological pathways underlying brain lateralization, handedness, and cognitive functions.