The present paper summarizes three experiments that investigate the effects of age of acquisition on first-language (L1) acquisition in relation to second-language (L2) outcome. The experiments use the unique acquisition situations of childhood deafness and sign language. The key factors controlled across the studies are age of L1 acquisition, the sensory–motor modality of the language, and level of linguistic structure. Findings consistent across the studies show age of L1 acquisition to be a determining factor in the success of both L1 and L2 acquisition. Sensory–motor modality shows no general or specific effects. It is of importance that the effects of age of L1 acquisition on both L1 and L2 outcome are apparent across levels of linguistic structure, namely, syntax, phonology, and the lexicon. The results demonstrate that L1 acquisition bestows not only facility with the linguistic structure of the L1, but also the ability to learn linguistic structure in the L2.
Dual-task automatization: The key role of sensory–motor modality compatibility
