Natural transmission of a mother tongue is the paradigm case in most considerations of language transmission. Much debate arises here as to whether a mental faculty unique to language is involved in the process. The generative linguistic view is that it does indeed involve an innate species capacity for language, or Universal Grammar, which Chomsky described as: ‘an element of the human biological endowment that appears to be subject to little variation apart from severe pathology’, in ‘Linguistics and adjacent fields’ (A. Kasher, The linguistic turn, Blackwell, 1991). By contrast, the traditionally popularised view of language transmission is that children learn language using ‘considerable intelligence’ after ‘many unsuccessful attempts’ and with ‘systematic help from their parents’, according to the psychologist Brian Butterworth (New Scientist vol. 100 p. 1380, 1983): no biological ‘mysterious helper’ in the form of a language faculty is required, from this research perspective.

A great deal of the argument revolves round the acquisition of formal features of language which have no salient semantic or pragmatic function, and which are therefore hard to motivate on extra-grammatical grounds. These are best illustrated by syntactic rules, such as gender marking and word order. During their third year children acquiring German and Swedish master a complex rule positioning verbs in second position in a main clause, but not in a subordinate clause. English-speaking children normally master by the age of four a rule requiring the presence of an auxiliary verb in interrogative and negative clauses, but not in affirmative clauses. Children acquiring Romance languages acquire gender marking early, and make few errors marking noun gender on definite articles, suggesting that the noun and its gender are acquired as a unit. Whether a noun is masculine or feminine has no communicative value, and the position of verbs and the presence of auxiliaries in clause structure is also not relevant to the communicative function of language.

Most researchers would agree that using language to communicate is a human species property. The debate focuses on how the formal features of grammar are acquired, insofar as they do not directly derive from communicative needs. The generative argument is that a general learning capacity would show difficulty with these features, whereas a mental capacity specifically dedicated to learning grammar, among other linguistic features, would explain the relatively early and relatively straightforward acquisition of formal grammatical features, without negative evidence, i.e. being told that a possible word order or gender assignment is incorrect. 

The opposing argument is that children’s mother tongue acquisition is supported by simplified parental modelling of the target structures, which helps children focus on them at an early stage. It is claimed that parental feedback allows children to discover what the appropriate forms of the target language are.

However, the notion of parental modelling being responsible for grammar acquisition has failed to win general consent. Hundreds of transcripts of pre-school age children interacting with their caregivers are now available online (http://childes.psy.cmu.edu), and show virtually no attempt by parents or others to ‘teach’ language to their children, certainly not as regards complex syntactic rules of which the parents themselves may not be consciously aware.

As things stand at present in the nature/nurture debate, there is a recognition of the importance both of what the child brings to the task of language acquisition, and of the ‘nurturant’ role of the language environment. The role of the caregiver is probably best viewed as being to contribute to a language-rich environment in which children’s own innate capacities are brought into play to construct their own cognitive system of the language of their speech community. The co-construction of language thanks to the joint contributions of child and linguistic environment is a crucial aspect of language transmission, as we will see below.

A related issue, also highly controversial, is whether the capacity for acquiring formal linguistic features accurately without instruction in childhood is subject to a time-limited ‘window of opportunity’. This is the so-called ‘critical period’ debate. Proponents of the critical period argue that many studies of second language acquisition show a declining ability with age to acquire the formal features of an L2. Its opponents claim that there exist many individuals with a perfect command of L2 grammar acquired after early childhood, so there can be no insuperable biological bar to later L2 grammar acquisition.

In the present state of play on this issue, it seems safest to adopt the view that language acquisition is facilitated by capacities available in early childhood, and that these, for one reason or another, become harder to recruit so as to attain a nativelike outcome. That ultimate attainment in L2 learning rarely attains L1 proficiency is a matter of common observation. The question is what prevents a nativelike outcome, and nature devises few if any experiments that allow us to pinpoint the factor responsible. One intervening factor is the presence of an L1 which may influence the acquisition of the L2 in various ways. Another is the type of linguistic environment available to the learner: Is it a situation of instructed learning, or does the learner have to ‘pick up’ the L2 by interaction with native speakers in professional or other social situations? In either case, the linguistic environment of the L2 learner clearly differs materially from the ‘nurturant’ context of caregiver input in the home, as with typical L1 acquisition.

Languages have the grammatical features that they do because they have been transmitted across many generations, despite never having been taught to mother tongue learners, in normal circumstances. They have survived what may be considered an ‘evolutionary’ challenge:  what is it that allows them to reproduce themselves in successive generations? Yet in another sense their survival is not an evolutionary one of functional selection among competitors (though see below). They are maintained if they are ‘noticeable’ by young pre-school children, that is, if they are detected as regularities in the linguistic environment.

Diachronically, this means that a language ‘has’ grammatical properties as long as they are successfully transmitted between generations. When we see instances of the process conventionally known as ‘language change’, such as the loss of subject-verb inversion in Old and Middle English, or the loss of Old English noun gender contrasts, we are looking at the effects of a grammatical feature having encountered a blockage in the transmission process. The language being used by caregivers at some point failed to exemplify the grammatical feature in question reliably enough for it to be transmitted. The matrix of L1 acquisition no longer ‘supported’ the maintenance of that particular grammatical feature, so it died.

Historically, contexts for language transmission in early childhood may have differed from our present-day model of the nuclear family. Current research I am doing on the transmission of French in medieval England suggests that up to the 14th century it was acquired in childhood in a context approximating natural acquisition. This concerned not only the nobility, but literate educated members of society, especially clerks. Hallmark features of early childhood acquisition may perhaps be seen in highly accurate noun gender marking, as reported on the page on Noun Gender Marking in Insular French legal texts.

Currently, research interest is focusing on what properties the linguistic environment needs to have to ensure the reliable transmission of a grammatical feature and avoid its demise. Frequency, saliency, and also the absence of a convenient alternative means of expressing the same notion, are factors that come into play. If there is competition from an alternative structure or form, functional selection of an evolutionary type may be seen as an appropriate way to view the process. In any case, there is no doubt that the language acquisition-language change domain crucially involves language use as a motor of language change, as well as appealing to the linguistic capacities of the learner. The transmission of language will be smooth when the learner gets regular straightforwardly assimilable input from caregivers, and problematic when the linguistic signals become mixed.  Exactly how to distinguish these types of linguistic environment is, in our present state of knowledge, a research goal still a long way off.