Some students of universal grammar study a variety of grammars to abstract generalizations called linguistic universals, often in the form of "If X holds true, then Y occurs." These have been extended to a range of traits, from the phonemes found in languages, to what word orders languages choose, to why children exhibit certain linguistic behaviors. as they considered issues of the Argument from poverty of the stimulus to arise from the constructivist approach to linguistic theory. The contrasting school of thought is known as functionalism.
Speakers proficient in a language know what expressions are acceptable in their language and what expressions are unacceptable. The key puzzle is how speakers should come to know the restrictions of their language, since expressions which violate those restrictions are not present in the input, indicated as such. This absence of negative evidence—that is, absence of evidence that an expression is part of a class of the ungrammatical sentences in one's language—is the core of the poverty of stimulus argument. For example, in English one cannot relate a question word like 'what' to a predicate within a relative clause (1):
(1) *What did John meet a man who sold?
Such expressions are not available to the language learners, because they are, by hypothesis, ungrammatical for speakers of the local language. Speakers of the local language do not utter such expressions and note that they are unacceptable to language learners. Universal grammar offers a solution to the poverty of the stimulus problem by making certain restrictions universal characteristics of human languages. Language learners are consequently never tempted to generalize in an illicit fashion.
Recent evidence suggests part of the human brain (crucially involving Broca's area, a portion of the left inferior frontal gyrus), is selectively activated by those languages that meet Universal Grammar requirements.
The presence of creole languages is cited as further support for this theory, especially by Bickerton's controversial Language bioprogram theory. These languages were developed and formed when different societies came together and were forced to devise their own system of communication. The system used by the original speakers was an inconsistent mix of vocabulary items known as a pidgin. When these speakers' children were acquiring their first language, they used the pidgin input to effectively create their own original language, known as a creole. Unlike pidgins, creoles have native speakers and make use of a full grammar.
The idea of universal grammar is supported by the creole languages by virtue of the fact that certain features are shared by virtually all of these languages. For example, their default point of reference in time (expressed by bare verb stems) is not the present moment, but the past. Using pre-verbal auxiliaries, they uniformly express tense, aspect, and mood. Negative concord occurs, but it affects the verbal subject (as opposed to the object, as it does in languages like Spanish). Another similarity among creoles is that questions are created simply by changing a declarative sentence's intonation, not its word order or content.
Some linguists oppose the universal grammar theory. Geoffrey Sampson maintains that universal grammar theories are not falsifiable and are therefore pseudo scientific theory, arguing that the grammatical generalizations made are simply observations about existing languages and not predictions about what is possible in a language.
Some feel that the basic assumptions of Universal Grammar are unfounded. Another way of defusing the poverty of the stimulus argument is if language learners notice the absence of classes of expressions in the input and, on this basis, hypothesize a restriction. This solution is closely related to Bayesian reasoning. Elman et al. argue that the unlearnability of languages assumed by UG is based on a too-strict, "worst-case" model of grammar.
James Hurford argues that the postulate of a "language acquisition device" essentially amounts to the trivial claim that languages are learnt by humans, and that the LAD isn't a theory so much as the explanandum looking for theories.
The Pirahã language has been claimed by the linguist Daniel Everett to be a counterexample to Universal Grammar, showing properties allegedly unexpected under current views of Universal Grammar. Among other things, this language is alleged to lack all evidence for recursion, including embedded clauses, as well as quantifiers and color terms. Some other linguists have argued, however, that some of these properties have been misanalyzed, and that others are actually expected under current theories of Universal Grammar. While most languages studied in that respect do indeed seem to share common underlying rules, research is hampered by considerable sampling bias. Linguistically, most diverse areas such as tropical Africa and America, as well as the diversity of Indigenous Australian and Papuan languages, have been insufficiently studied. Furthermore, language extinction apparently has affected those areas most where most examples of unconventional languages have been found to date.