argue that certain components of language are resilient in the face of either environmental or organic deviations from the typical language-learning circumstances, while other components of language are relatively fragile (Goldin-Meadow, 1982). For example, clinical notes on language development in children who have been adopted from institutions suggest that despite becoming proficient in the language of their new homes, these children may not use language as readily for expressing emotion, requesting aid from adults, or expressing ideas and fantasy (Provence and Lipton, 1962). It is not known if they are as likely as other children to use language to guide problem solving, although this might be one reason for their poorer executive functioning (Gunnar, in press).

Perhaps the most dramatic example of how language is vulnerable to environmental influences concerns the role of the timing of language inputs in language proficiency. This literature is highly relevant to current debates about critical or sensitive periods in development. There is, in fact, a considerable amount of evidence suggesting that early exposure to a language results in greater proficiency in that language than late exposure. For example, deaf children of hearing parents, as mentioned earlier, are typically not exposed to a conventional sign language at birth and may not receive their first exposure to such a system until adolescence or later. These individuals thus provide an excellent “experiment of nature” to test the effects of learning a first language at varying times in the life course. Findings from these studies suggest that certain aspects of language—morphological properties, for example, which involve how smaller parts of words make up bigger words and affect word meaning (e.g., “eat” + “ing” = “eating”)—are affected by the age at which the learner is first exposed to sign language. An example of a morphological property in sign is movement added to a sign such as “eat” to create the meaning “eat continuously over time.” Late learners, although perfectly capable of conversing in sign, do not have complete productive control over many of the complex morphological properties of the language (Newport, 1991). Interestingly, however, certain properties of language—such as the order of signs in a sentence—appear to be completely unaffected by the age at which the learner is first exposed to the language. In other words, native-like competence is possible for sign order whether or not the learner is exposed to sign early in life—but is far less likely for morphological properties.

Similar patterns arise in second-language learning (Newport, 1991). Learners who are first exposed to their second language after puberty find that certain aspects of that language (often morphological aspects) are difficult, if not impossible, to master even after decades of use, while others (like word order) are relatively easy to control. For example, learning to systematically produce endings such as “-ed” in “walked,” which adds the past meaning, or “s” in “shoes,” which adds the plural meaning, is far more

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