As we noted in the introduction to this Topic, language acquisition is a remarkable achievement that most children appear to accomplish with ease. For some children, however, language acquisition is made more difficult by physical or cognitive impairments. In this section we briefly discuss two special populations within which language acquisition follows a different pathway than it does in most children. Both of these cases illustrate important points about language acquisition in general.

Deaf children have an obvious disadvantage when it comes to learning spoken language: they can't hear it. For children who are born with complete hearing loss, the journey to spoken language production and comprehension is difficult if not impossible. However, there is another pathway toward language that illustrates the remarkable plasticity of the human brain: sign language. Sign language is a complete linguistic system--or, more precisely, a variety of linguistic systems, some of which differ from each other as much as English does from Japanese--based on gestures of the face, arms, hands, and upper body. Children who are raised in environments where they are frequently exposed to sign language learn it just as rapidly and successfully as hearing children learn spoken language. In some cases, they may even learn to express basic signed words earlier than hearing children produce spoken words. Even more amazing, there is evidence that deaf children who are raised together in the absence of an established signed language come to develop one of their own. What is the significance of this for our understanding of language acquisition more generally? First, language is not tied to audition and vocal production; it can function just as well through the medium of vision and gesture. Second, even in the absence of linguistic input children appear to have a drive towards language acquisition--what Steven Pinker has called, in his book of the same name, "the language instinct."

Williams syndrome is an extremely rare genetic disorder that is characterized by "elfin" facial characteristics, mental retardation (IQs in the range of 40-80), sociability, and high levels of linguistic functioning. This strange mixture of abilities-- particularly the paradox of very low cognitive functioning and very high linguistic functioning--has led some researchers to view Williams syndrome children as evidence for a specific, genetically-determined language module. A more moderate assessment of the evidence suggests that Williams syndrome children show an abnormal pattern of linguistic functioning that may reflect preserved cognitive abilities in a few select areas-- just those areas that are crucial for language acquisition and production. What can we learn about language acquisition from children with Williams syndrome? First, language and cognition are not inextricably bound together; it is likely that certain cognitive functions are necessary for language but that others are completely irrelevant. Second, genes play an important role in providing children with the basic machinery for language acquisition.