Answer: The properties of language development — a spontaneous maturational development typical of the human species as a whole —strongly suggest that the linguistic capacity is part of the genetic endowment of human beings. The most significant contribution of generative theory to the psychology of language is that there must be some innate biological representation of the abstract structure of language built into the human nervous system. Lennberg (1960, 1967) has said this predisposition has been shaped by evolution. It is, he points out, comparable to man’s bipedal g.ait. From this point of view, all children are biologically programmed with the capacity to develop language- namely, the language(s) they are significantly exposed to during the appropriate maturational stage. Language development can, thus be regarded as analogous to other biological developments in human growth and maturation.
The most ambitious claim in, support of the biological basis of language capacity is that there is a critical period for language acquisition, namely, between 18 months and early puberty in most people. Within this period, language acquisition is expected to proceed normally, but outside it, language acquisition is difficult, if not impossible.
The evidence for a biological basis- for a specifically linguistic ability was most enthusiastically presented by Lenneberg (1967, 1969). He drew a suggestive parallel between the time course of certain features of the maturation of the human brain, and behavioral characteristics of language development under unusual condition of learning. There are two hemispheres to the brain, connected to each other by a structure known as the corpus callosum. At birth, the hemispheres are not structurally identical, but Lenneberg assumed that they were functionally identical and were each capable in the beginning of supporting language development. Around 18 months he assumed that the hemispheres began to become functionally specialized in that each hemisphere began to control different areas of human activity. In particular, in the normal person, the left hemisphere began to control functions characteristic of language. By puberty, Lenneberg assumed that the functional specialization had become fixed, so that if a person had not developed language, his left hemisphere would have taken over control of other functions, and language acquisition would be difficult, if not impossible.
Consistent with this account were the following claims: Brain damage to the left hemisphere in adults led to aphasia in 70 percent of cases surveyed by Lenneberg, while brain damage to children below 18 months had the same effect on their language development, whether damage was to the right or left hemisphere. For children between 2 and 4 years, recovery of the language function generally possible after a variable period of disruption, during which was the child appeared to run swiftly through the stages of development he had passed before the injury. It was claimed that people who have not learned to speak by puberty were unlikely to do so, and learning a foreign language is both harder and follows a different learning pattern after puberty than before. Children who become deaf before they have began to speak have as great a difficulty in learning a form of language as children born deaf, but those who have learned some language and lose their hearing around the age of 3 and 4 can be trained more quickly and more successfully in language when they start school than congenitally deaf children.
There is now evidence that a certain degree of functional specialization of the hemispheres can be detected long before the child is 18 months old. Molfese (1977) measured the auditory evoked potential form both hemispheres of groups of infants, children and adults, to speech stimuli and non-speech stimuli which came from a loudspeaker placed above the subject. He found that all groups showed differential hemispherical responding to these materials and that, in fact, both speech and non- speech were lateralized more strongly in his infants than in his adults.
It seems, then, that cerebral specialization of function may begin long before the onset of language acquisition. There is probably still a complex development of cerebral specialization awaiting the infant, but it seems that the strong view, that before 18 months both hemispheres are equally capable of supporting language development, has to be modified. At the other end of the critical period, likewise, the picture is more complicated than Lenneberg would lead us to suppose. Curtiss (1977) reports a recent case where a girl, Genie, who began to learn her first language after puberty made some progress. In this case, however, the girl appeared to use her right hemisphere for language and the course of development was different in many ways from that of children acquiring language under normal conditions. Curtiss interprets these results as indicating that there are constraints on the nature of language acquisition beyond the critical period.
Lenneberg makes four distinct points in arguing the similarity between the evolutionary status of language and that of bipedal gait.
1. There is no variation within the species in the fundamental characteristics of language- all human language is vocal and all human languages make about the same numbers of phonetic distinction. There are, in all languages, roughly forty different phonemes.
2. All languages show concatenation which always obeys syntactic principles.
3. There is an organic correlate of language. That organic correlate is reflected in uniformity and universal characteristics of language. Children appear to learn languages in much the same way, no matter what the culture. There is no history of the development of language within the species. Language change, but they do not become more complex or simpler. They simply change. Language seems to be unique in its universality and complexity and stability. The concatenation system by which man represents language is as historically static as is man’s bipedal gait.
Lenneberg (1966) also discusses the question of language as a specific skill. But Lenneberg does not unlock a specific ability for language from other cognitive abilities.
In this way the traditional view that language is unique to human beings may have a sound biological basis. Just as other biological characteristics can be unique to a certain species, so too the capacity for language and other properties of human mental functioning may well be as unique part of the genetic endowment of human being.
Snow and Hoefnagle-Hohle (1978) have challenged the view that acquisition of a second language is more efficient during the critical period than after it. Thus Lenneberg’s proposal of a clear critical period for language acquisition has to be considerably modified. A certain degree of functional specialization of the brain does take place before the onset of language, and so specific brain damage during this period can have effects on later language development. Acquisition of a second language after the critical period may be quite efficient and acquisition of first language may still be possible. The pattern of development during childhood and adolescence may depend critically on the information-processing skills available to the child. However, we should conclude from these studies that not the biological factors are irrelevant to language-learning ability, but rather the relation between language acquisition and its biological basis is more elusive and tantalizing than earlier accounts indicated.