are delayed in language learning relative to mental age (Fowler et al., 1994). Yet children with Williams syndrome (a rare metabolic disorder), who are as mentally retarded in terms of IQ as children with Down syndrome, display considerably better grammatical skills (Bellugi et al., 1988). Thus, low intelligence does not, in all cases, preclude grammatical development.
The inverse is true, as well: language difficulties do not inevitably imply cognitive difficulties. For example, children with specific language impairment, by definition, have no cognitive disabilities but do have difficulty learning language. As a final piece of evidence, adults, who are cognitively mature, typically have difficulty learning a second language (Johnson and Newport, 1989), suggesting that cognitive maturity is not sufficient to guarantee grammatical development (and after some sensitive period may even become an impediment, as discussed below). In general, in fact, the growth of cognitive, language, and literacy skills is much more domain-specific, constrained, and modular than previously thought (Christian et al., in press). A similarly complex pattern holds for social skills. For example, children with Down syndrome are relatively adept socially (in comparison to children with autism) yet have difficulty learning grammar (Fowler et al., 1994). In contrast, autistic children's social interactions are atypical, yet when they are able to learn language, their grammatical skills are intact (Tager-Flusberg, 1994).
Language learning is robust in the same way that developing an attachment to a caregiver is robust. Only in aberrant conditions of care, such as extreme neglect or institutional deprivation, do children fail to form attachments to anyone (see Chapter 9). However, not all infants develop secure attachments—secure attachments are formed in a more restricted set of circumstances. Similarly, children acquire language with very little environmental support (deaf children inventing their own gesture systems are a good example). However, the specific language that they learn and certain qualities of their language depend on specific features of the environment in which they learn language. And these aspects of language are often instrumental to subsequent cognitive and social growth. Children can be at risk in society, not because they do not have mastery of a language, but because they do not have complete mastery of the dominant language of their society, particularly at the time of formal school entry.
It is important to recognize that language is not a unitary phenomenon. Certain aspects of language may turn out to be more susceptible to varia tions in learning conditions (both internal and external) than others. If, across a variety of exceptional circumstances, the same components of language tend to be delayed while others remain intact, one might begin to