Language acquisition research, whether in the area of first or second language acquisition, is concerned with understanding the relationship between the kinds of input that the language learner is exposed to, and the kind of linguistic knowledge they end up with, and which they can use in language processing (comprehension and production of spoken language, reading and writing); also, perhaps, in complex thought.

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Almost everyone involved in linguistic research accepts that there is a important theoretical distinction between implicit knowledge of language (which generativists call grammatical competence) and language use (performance): the ideological divisions and (often vitriolic) debates surround the questions of how to define and how best to investigate competence, and whether one or other concept—competence or performance—should be considered to be of any deep scientific interest. Many generativists take the view that performance/language use is at best of secondary importance, and that the everyday notion of a language (e.g., 'The English Language') is poorly defined and theoretically incoherent. By contrast, most functionalists, cognitive linguists, psychologists and anthropologists consider that language acquisition and use can only be properly understood within a theory (or set of theories) that takes into account the social, biological, physiological and cognitive structures within which language is embedded, and that it is meaningless to study competence without regard to actual performance ability; see, for example, Seidenberg & MacDonald (1999).

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As someone who has worked for and with language researchers on both sides of the formal/functional divide, I have a good appreciation of the ways our ideologies frame and constrain the questions we ask about competence, and the ways through which we advance our various discourses. In a recent overview article on psycholinguistics, which has now been accepted as the core content of a new textbook, I attempt to guide myself and the reader through what I term 'the two souls of psycholinguistic theory', in order to try to understand some of the main empirical results in a more ecumenical context. I also try to sketch a particular view of competence in two recent publications (Duffield 2003, 2004).

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Having thought it over, it seems to me that the empirical crux of the matter in the unfortunately named 'Nature-Nuture' debate is grammatical convergence: whether adult native-speakers of a particular language variety, from different socio-economic groups, varying in measures of general intelligence, working memory and educational attainment, with differences in infant and childhood development, nevertheless end up with essentially identical competence grammars. (This is not the question of whether everyone uses language equally well: no experiments are needed to determine that we're all quite different in this regard.) If we can show that native-speakers really do converge in this way, it reinforces the generativist position against the emergentist one. If however the behavioral evidence for non-convergence is stronger, this would seem to render moot the nativist "Poverty of Stimulus" arguments (cf. Pullum & Scholz 2002, Reali & Christiansen 2004).

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What is most striking about this is that we still really don't have any clear answers, because the work hasn't been done: generativists simply assume convergence without clear empirical evidence, while emergentists up to now have not been interested in investigating the sorts of phenomena that tell us about implicit competence, preferring to concentrate on production data instead. (Whatever it might say about differences in language performance, no amount of spontaneous production data will tell us much about convergence/non-convergence in abstract grammatical competence). I address this issue a little more fully in a recent commentary article ('The kids are alright...aren't they?').

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Theses questions are also addressed in my new book Perspectives on Psycholinguistic Theories: Raiding the Inarticulate (CUP). Available from CUP, and from Amazon (search Duffield, Reflections).