Literacy improves the comprehension of object relatives.

While there is a considerable body of research indicating that the acquisition of literacy has profound effects on many aspects of language and cognition, to our knowledge, very little is known about its effects on morphosyntax. In this paper, we explore the effects of literacy on the comprehension of Spanish object relative clauses, a structure which is typically acquired by literate children about the age of 10, i.e., after a considerable amount of exposure to written language. We tested three groups of native Spanish speakers (semi-literates, late-literates and high-literates) using a picture selection task. Subject relatives were used as a control condition. All three groups performed at ceiling on subject relatives (group means of 95% or above). In contrast, we observed very large differences in performance on object relatives, with the semi-literate group performing at chance (51% correct) and the late-literate group slightly above chance (65% correct). Performance in the high-literate group was much better, although not quite at ceiling (82% correct). The results appear to support the hypothesis that literacy helps in the acquisition of some aspects of grammar. This could be partly due to differences in IQ, metalinguistic awareness, working memory and/or executive functioning. The results are also consistent with the 'training wheels' hypothesis (Dąbrowska, 2020), according to which the availability of written representations facilitates the acquisition of difficult structures by easing memory load and enabling speakers to process sentences at their own pace.