In linguistics, the innateness hypothesis, also known as the nativist hypothesis, holds that humans are born with at least some knowledge of linguistic structure. On this hypothesis, language acquisition involves filling in the details of an innate blueprint rather than being an entirely inductive process. The hypothesis is one of the cornerstones of generative grammar and related approaches in linguistics. Arguments in favour include the poverty of the stimulus, the universality of language acquisition, as well as experimental studies on learning and learnability. However, these arguments have been criticized, and the hypothesis is widely rejected in other traditions such as usage-based linguistics. The term was coined by Hilary Putnam in reference to the views of Noam Chomsky.

Linguistic nativism is the hypothesis that humans are born with some knowledge of language. It is intended as an explanation for the fact that children are reliably able to accurately acquire enormously complex linguistic structures within a short period of time. The central argument in favour of nativism is the poverty of the stimulus. Additional arguments come from the fact that language acquisition among children occurs in ordered developmental stages and that adult learners – having passed the critical age for language acquisition – are typically unable to acquire native-like proficiency in a second language.

Poverty of the stimulus arguments claim that the evidence a child receives during language acquisition is not sufficient to determine the eventual linguistic output. For instance, in one such argument formulated by Noam Chomsky, he argued that children's experiences with polar questions in languages such as English language would not favor the actual subject–auxiliary inversion rule over a hypothetical one which pertains to linear order rather than hierarchical structure.

Pullum and Scholz summarised the properties of a child's environment. They identified properties of positivity, degeneracy, incompleteness and idiosyncrasy. Under positivity, they assert that children are only exposed to positive linguistic data. Moreover, there is lack in negative data that aids a child in identifying ungrammatical sentences that are unacceptable in the language. It is also claimed^{[by whom?]} that children are unable to acquire a language with positive evidence alone. In addition, under degeneracy, it is stated that children are often exposed to linguistic data that are erroneous. This is supported by Zohari, who states that in adult speech, erroneous utterances that include speech slips, ungrammatical sentences, incomplete sentences, etc. are often observed. Furthermore, the linguistic data each child is exposed to is different (i.e. idiosyncrasy) and there are many utterances that a child might not have heard (i.e. incompleteness). However, despite the properties mentioned above, children would eventually be able to deliver a linguistic output that is similar to the target language within a relatively short amount of time. In contrast, when placed in certain environments, other organisms are unable to attain the language mastery humans have reached.

The validity of poverty of stimulus arguments is not universally accepted, and is the subject of ongoing debate.

The critical-period hypothesis of the linguist Eric Lenneberg states that full native competence in acquiring a language can only be achieved during an optimal period. This hypothesis supports the innateness hypothesis about the biological innateness of linguistic competence. Lenneberg expressed that age plays a salient role in the ability to acquire language. According to him, a child before the age of two will not sufficiently acquire language, while development of full native competence in a language must occur before the onset of puberty. This suggests that language is innate and occurs through development instead of through feedback from the environment. As a result, should a child not hear any language during this period, the child would not be able to learn nor be able to speak. This hypothesis is also said^{[by whom?]} to explain why adults do not acquire languages as well as children.

The case of the feral child Genie provides evidence for the critical-period hypothesis. When discovered, she was without language. Genie's subsequent language-acquisition process was studied, whereby her linguistic performance, cognitive and emotional development was deemed abnormal. Genie was said^{[by whom?]} to have right-hemisphere language, resembling other cases where language was acquired outside of the "critical period". This would lend support to Lenneberg's hypothesis. Moreover, some saw the case of Genie as a support to the innateness hypothesis. When the LAD is not triggered during the critical period, the natural process of language acquisition cannot be reached. However, Genie's case is complex and controversial. It has been argued^{[by whom?]} that it does not support linguistic innateness. Some^[which?] have asserted that there is at least a possible degree of first-language acquisition beyond the critical period. Moreover, emotional and cognitive deprivation may have also played a part in Genie's linguistic and cognitive difficulties.

The development of the Nicaraguan sign language (NSL) by students in a school for the deaf also lends evidence to the critical-period hypothesis. Initially a pidgin sign language with simple grammar, it had large grammatical differences and variations across signers. Eventually, the pidgin became a full-fledged language (like a creole) as younger signers developed a significantly more grammatically-structured and regular system such as specific grammatical structures Often, the differences in abilities between younger and older students learning to use sign language are said to suggest evidence for a critical period. The spontaneity of the development of NSL also suggests that there is an innate element to the process of language learning.