Poverty of the stimulus

The poverty of the stimulus (POTS) argument is a variant of the epistemological problem of the indeterminacy of data to theory that claims that grammar is unlearnable given the linguistic data available to children. As such, the argument strikes against empiricist accounts of language acquisition. Inversely, the argument is usually construed as in favour of linguistic nativism because it leads to the conclusion that knowledge of some aspects of grammar must be innate. Nativists claim that humans are born with a specific representational adaptation for language that both funds and limits their competence to acquire specific types of natural languages over the course of their cognitive development and linguistic maturation. The basic idea informs the teachings of Socrates, Plato, and the Pythagoreans, pervades the work of the Cartesian linguists and Wilhelm von Humboldt, and surfaces again with the contemporary linguistic theories of Noam Chomsky. The argument is now generally used to support theories and hypotheses of generative grammar. The name was coined by Chomsky in his work Rules and Representations (Chomsky, 1980). The thesis emerged out of several of Chomsky's writings on the issue of language acquisition. The argument has been persuasive within linguistics, forming the empirical backbone for the theory of universal grammar. It is taught to students in most linguistics and psycholinguistics courses. Despite a body of criticism it remains popular amongst linguists.

Summary of the argument
Though Chomsky reiterated the argument in a variety of different manners, one common structure to the argument can be summed up as follows:


 * Premises:
 * 1) There are patterns in all natural languages (i.e. human languages) that cannot be learned by children using positive evidence alone.  Positive evidence is the set of grammatical sentences the language learner has access to, that is, by observing the speech of others.  Negative evidence, on the other hand, is the evidence available to the language learner about what is not grammatical.  For instance, when a parent corrects a child's speech, the child acquires negative evidence.
 * 2) Children are only ever presented with positive evidence for these particular patterns. For example, they only hear others speaking using sentences that are "right", not those that are "wrong".
 * 3) Children do learn the correct grammars for their native languages.
 * Conclusion: Therefore, human beings must have some form of innate linguistic capacity which provides additional knowledge to language learners.

Evidence for the argument
The validity of the argument itself is unquestioned. Very few people, if any, would argue that Chomsky's conclusion doesn't follow from his premises. Thus, anyone who accepts the first three propositions must accept the conclusion. Many linguists accept all of the premises and consider there to be quite a bit of evidence for them.

Several patterns in language have been claimed to be unlearnable from positive evidence alone. One example is the hierarchical nature of languages. The grammars of human languages produce hierarchical tree structures and some linguists argue that human languages are also capable of infinite recursion (see Context-free grammar). For any given set of sentences generated by a hierarchical grammar capable of infinite recursion there are an indefinite number of grammars which could have produced the same data. This would make learning any such language impossible. Indeed, a proof by E. Mark Gold showed that any formal language which has hierarchical structure capable of infinite recursion is unlearnable from positive evidence alone. (Of course, this is "unlearnable" in the sense that it is impossible to formulate a procedure which will discover with certainty the correct grammar. It may well be possible to arrive at a good approximation of the correct grammar &mdash; or even the correct grammar itself &mdash; by a mixture of well-tuned heuristics, luck, etc.)

Another example of language pattern claimed to be unlearnable from positive evidence alone is subject-auxiliary inversion in questions, i.e.:


 * You are happy.
 * Are you happy?

There are two hypotheses the language learner might postulate about how to form questions: (1) The first auxiliary verb in the sentence (here 'are') moves to the beginning of the sentence, or (2) the 'main' auxiliary verb in the sentence moves to the front. In the sentence above, both rules yield the same result since there is only one auxiliary verb. But, you can see the difference in this case:


 * Anyone who is interested can see me later.
 * 1) Is anyone who interested can see me later?
 * 2) Can anyone who is interested see me later?

Of course, the result of rule (1) is ungrammatical while the result of rule (2) is grammatical. So, rule (2) is (approximately) what we actually have in English, not rule (1). The claim, then, first is that children don't see sentences as complicated as this one enough to witness a case where the two hypotheses yield different results, and second that just based on the positive evidence of the simple sentences, children could not possibly decide between (1) and (2). Moreover, even sentences such as (1) and (2) are compatible with a number of incorrect rules (such as "front any auxiliary), as noted by Lasnik and Uriagereka (2002). Thus, if rule (2) was not innately known to infants, we would expect half of the adult population to use (1) and half to use (2). Since that doesn't occur, rule (2) must be innately known.  (See Pullum 1996, linked below, for the complete account and critique.)

The last premise, that children successfully learn language, is considered to be evident in human speech. Though people occasionally make mistakes, human beings rarely speak ungrammatical sentences, and generally do not label them as such when they say them. (Ungrammatical in the descriptive sense, not the prescriptive sense.)

That many linguists accept all three of the premises is testimony to Chomsky's influence in the discipline, and the persuasiveness of the argument. Nonetheless, the APS has many critics, both inside and outside linguistics.

Criticisms of the argument
Though some recognize the theory as valid, the soundness of the poverty of stimulus argument is widely questioned. Indeed, every one of the three premises of the argument has been questioned at some point in time. Much of the criticism comes from researchers who study language acquisition and computational linguistics. Additionally, connectionist researchers have never accepted most of Chomsky's premises, because these premises are at odds with connectionist beliefs about the structure of cognition.

The first and most common critique, is that positive evidence is actually enough to learn the various patterns which linguists claim are unlearnable by positive evidence alone. A common argument is that the brain's mechanisms of statistical pattern recognition could solve many of the imagined difficulties. For example, researchers using neural networks and other statistical methods have programmed computers to learn rules such as (2) cited above, and have claimed to have successfully extracted hierarchical structures, all using positive evidence alone. (See Bates & Elman (1996), and Solan et al. (2005) below.) Some linguists, such as Steven Pinker, remain skeptical as to whether these techniques will ever extract anything other than "toy" grammars. 

As for the argument based on Gold's proof, it's not clear that human languages are truly capable of infinite recursion. Clearly, no speaker can ever in fact produce a sentence with an infinite recursive structure, and in certain cases (for example, center embedding), people are unable to comprehend sentences with only a few levels of recursion. Chomsky and his supporters have long argued that such cases are best explained by restrictions on working memory, since this provides a principled explanation for limited recursion in language use. Some critics argue that this removes the falsifiability of the premise. Returning to the big picture, it is questionable whether Gold's research actually bears on the question of natural language acquisition at all, since what Gold showed is that there are certain classes of formal languages for which some language in the class cannot be learned given positive evidence alone. It's not at all clear that natural languages fall in such a class, let alone whether they are the ones that are not learnable. (See Johnson 2004). Probably for these reasons, Chomsky himself has never advocated an argument based on Gold's proof.

There is also criticism about whether negative evidence is really so rarely encountered by children. Pullum (1996) (linked below) argues that learners probably do get certain kinds of negative evidence. In addition, if one allows for statistical learning, negative evidence is abundant. Consider that if a language pattern is never encountered, but its probability of being encountered would be very high were it true, then the language learner might be right in considering absence of the pattern as negative evidence. Chomsky accepts that this kind of negative evidence plays a role in language acquisition, terming it "indirect negative evidence", though he does not think that indirect negative evidence is sufficient for language acquisition to proceed without Universal Grammar. In Pullum and Scholz (2002) it is claimed that examples of a number of purportedly rare constructions are reasonably common in available corpora of written texts. This claim was subsequently challenged in Legate and Yang (2002), who find that these constructions are indeed exceedingly rare in those corpora that contain transcriptions of speech addressed to children. Other responses to the article by Pullum and Scholz have questioned the relevance of their result, given that children learn from spoken language, and may not have sufficiently good memory or attention to be able to learn reliably from a small number of sentences. Pullum and Scholz respond that their corpus analysis is "preliminary", intended to serve as an impetus for further research, rather than a decisive refutation of any particular Poverty of Stimulus argument.

Finally, it has been argued that people may not learn the exact same grammars as each other. If this is the case, then only a weak version of the third premise is true, as there would be no fully "correct" grammar to be learned. However, in many cases, Poverty of Stimulus arguments do not in fact depend on the assumption that there is only one correct grammar, but rather that there is only one correct class of grammars. For example, the Poverty of Stimulus argument from question formation depends only on the assumption that everyone learns a structure-dependent grammar.