Humans are unique in their ability for language and a strong population-wide right-hand preference for object manipulation. A number of researchers (e.g., Arbib, 2005; Crow, 2002) suggest that an association between language asymmetry and handedness was crucial for human evolution and development. However, developmental studies on language and handedness association demonstrate mixed results. Importantly, only a small number of developmental studies addressed handedness-language relations in adults. Moreover, the majority of studies on handedness and language relations rely on homogeneous samples of right-handed monolingual English speakers. To this day it is not known whether the results of such studies can be extrapolated to bilingual people, and whether results obtained from studies with children can be extrapolated to adults. The current study is the first of its kind systematically examining handedness and language in a sample of over 1,800 participants with diverse language background (over 50 different languages). The study examined handedness and language asymmetry in monolinguals, early bilinguals (acquiring a second language before age 6) and late bilinguals (acquiring a second language after age 6). Additional parameters such as motor asymmetry (a preference for right footedness) and gender were also examined for potential effects on asymmetry formation in all participants. Finally, a subsample of monolingual and bilingual participants was examined on asymmetry of a gesture and object manipulation. Study results suggest that contrary to previous claims of language asymmetry and handedness association, they are not strongly related in adults. Language asymmetry and the age of the second language acquisition predicted only a small portion of handedness score. Footedness and gender were stronger predictors of handedness. Females exhibited stronger asymmetry than males; more right-footed participants tended to be more right-handed. Contrary to studies with children, current study adult participants were more strongly lateralised for object manipulation than for gesture. In conclusion, the current study suggests that handedness and language relations are dynamic in development; that their relations are not as robust as was previously suggested; and finally, that the research field of handedness-language relations would benefit from diversifying study samples.
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Thesis advisor: Racine, Timothy
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