The ability to process the linguistic input in real time is crucial for successfully acquiring a language, and yet little is known about how language learners comprehend or produce language in real time. Against this background, we have conducted a detailed study of grammatical processing in language learners using experimental psycholinguistic techniques and comparing different populations (mature native speakers, child first language [L1] and adult second language [L2] learners) as well as different domains of language (morphology and syntax). This article presents an overview of the results from this project and of other previous studies, with the aim of explaining how grammatical processing in language learners differs from that of mature native speakers. For child L1 processing, we will argue for a continuity hypothesis claiming that the child's parsing mechanism is basically the same as that of mature speakers and does not change over time. Instead, empirical differences between child and mature speaker's processing can be explained by other factors such as the child's limited working memory capacity and by less efficient lexical retrieval. In nonnative (adult L2) language processing, some striking differences to native speakers were observed in the domain of sentence processing. Adult learners are guided by lexical-semantic cues during parsing in the same way as native speakers, but less so by syntactic information. We suggest that the observed L1/L2 differences can be explained by assuming that the syntactic representations adult L2 learners compute during comprehension are shallower and less detailed than those of native speakers.Assigning a grammatical structure to an input string presupposes knowledge of the combinatorial rules and grammatical constraints that apply in the language being processed. At the same time, however, successful grammar building presupposes the availability of appropriate mechanisms for processing the linguistic input (compare Chaudron, 1985;Fodor, 1998aFodor, , 1998bFodor, , 1999Valian, 1990). This apparent acquisition paradox poses a challenge for theories of first (L1) and second language (L2) acquisition that requires our existing knowledge of language learners' grammatical development to be supplemented by a detailed and systematic investigation of their grammatical processing routines. Although several decades' worth of psycholinguistic research has greatly increased our understanding of how mature readers and listeners process their native language in real time, psycholinguistically informed research into language learners' processing mechanisms

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Four groups of second language (L2) learners of English from different language backgrounds (Chinese, Japanese, German & Greek) and a group of native speaker controls participated in an on-line reading-time experiment with sentences involving long-distance wh-dependencies. While the native speakers showed evidence of making use of intermediate syntactic gaps during processing, the L2 learners appeared to associate the fronted wh-phrase directly with its lexical subcategoriser, regardless of whether or not the subjacency constraint was operative in their native language. This finding is argued to support the hypothesis that L2 learners under-use syntactic information in L2 processing, which prevents them from processing the L2 input in a native-like fashion.45

This study investigates the way adult second language (L2) learners of English resolve relative clause attachment ambiguities in sentences such as The dean liked the secretary of the professor who was reading a letter. Two groups of advanced L2 learners of English with Greek or German as their first language participated in a set of off-line and on-line tasks. The results indicate that the L2 learners do not process ambiguous sentences of this type in the same way as adult native speakers of English do. Although the learners' disambiguation preferences were influenced by lexical–semantic properties of the preposition linking the two potential antecedent noun phrases (of vs. with), there was no evidence that they applied any phrase structure–based ambiguity resolution strategies of the kind that have been claimed to influence sentence processing in monolingual adults. The L2 learners' performance also differs markedly from the results obtained from 6- to 7-year-old monolingual English children in a parallel auditory study, in that the children's attachment preferences were not affected by the type of preposition at all. We argue that children, monolingual adults, and adult L2 learners differ in the extent to which they are guided by phrase structure and lexical–semantic information during sentence processing.

The core idea that we argued for in the target article was that grammatical processing in a second language (L2) is fundamentally different from grammatical processing in one's native (first) language (L1). Our major source of evidence for this claim comes from experimental psycholinguistic studies investigating morphological and syntactic processing in child and adult native speakers, and nonnative speakers who acquired their L2 after childhood and for whom their L1 is the dominant language. With respect to child L1 processing, we argued for acontinuity of parsing hypothesisclaiming that the child's structural parser is basically the same as that of mature speakers and does not change over time. Adult L2 learners, in contrast, were seen to underuse syntactic information during sentence processing and to rely more on lexical–semantic cues to interpretation. To account for the observed L1/L2 differences in processing, we proposed the shallow structure hypothesis (SSH) according to which the representations adult L2 learners compute during processing contain less syntactic detail than those of child and adult native speakers.