Statistical language acquisition, a branch of developmental psycholinguistics, studies the process by which humans develop the ability to perceive, produce, comprehend, and communicate with natural language in all of its aspects (phonological, syntactic, lexical, morphological, semantic) through the use of general learning mechanisms operating on statistical patterns in the linguistic input. Statistical learning acquisition claims that infants' language-learning is based on pattern perception rather than an innate biological grammar. Several statistical elements such as frequency of words, frequent frames, phonotactic patterns and other regularities provide information on language structure and meaning for facilitation of language acquisition.

Philosophy Fundamental to the study of statistical language acquisition is the centuries-old debate between rationalism (or its modern manifestation in the psycholinguistic community, nativism) and empiricism, with researchers in this field falling strongly in support of the latter category. Nativism is the position that humans are born with innate domain-specific knowledge, especially inborn capacities for language learning. Ranging from seventeenth century rationalist philosophers such as Descartes, Spinoza, and Leibniz to contemporary philosophers such as Richard Montague and linguists such as Noam Chomsky, nativists posit an innate learning mechanism with the specific function of language acquisition.Russell, J. (2004). What is Language Development?: Rationalist, Empiricist, and Pragmatist Approaches to the Acquisition of Syntax. Oxford University Press.

In modern times, this debate has largely surrounded Chomsky's support of a universal grammar, properties that all natural languages must have, through the controversial postulation of a language acquisition device (LAD), an instinctive mental 'organ' responsible for language learning which searches all possible language alternatives and chooses the parameters that best match the learner's environmental linguistic input. Much of Chomsky's theory is founded on the poverty of the stimulus (POTS) argument, the assertion that a child's linguistic data is so limited and corrupted that learning language from this data alone is impossible. As an example, many proponents of POTS claim that because children are never exposed to negative evidence, that is, information about what phrases are ungrammatical, the language structure they learn would not resemble that of correct speech without a language-specific learning mechanism.Chomsky, N. (1965). Aspects of the Theory of Syntax. Cambridge, MA: MIT Press. Chomsky's argument for an internal system responsible for language, biolinguistics, poses a three-factor model. "Genetic endowment" allows the infant to extract linguistic info, detect rules, and have universal grammar. "External environment" illuminates the need to interact with others and the benefits of language exposure at an early age. The last factor encompasses the brain properties, learning principles, and computational efficiencies that enable children to pick up on language rapidly using patterns and strategies.

Standing in stark contrast to this position is empiricism, the epistemological theory that all knowledge comes from sensory experience. This school of thought often characterizes the nascent mind as a tabula rasa, or blank slate, and can in many ways be associated with the nurture perspective of the "nature vs. nurture debate". This viewpoint has a long historical tradition that parallels that of rationalism, beginning with seventeenth century empiricist philosophers such as Locke, Bacon, Hobbes, and, in the following century, Hume. The basic tenet of empiricism is that information in the environment is structured enough that its patterns are both detectable and extractable by domain-general learning mechanisms. In terms of language acquisition, these patterns can be either linguistic or social in nature.

Chomsky is very critical of this empirical theory of language acquisition. He has said, "It's true there's been a lot of work on trying to apply statistical models to various linguistic problems. I think there have been some successes, but a lot of failures." He claims the idea of using statistical methods to acquire language is simply a mimicry of the process, rather than a true understanding of how language is acquired.

Experimental paradigms

Headturn Preference Procedure (HPP) One of the most used experimental paradigms in investigations of infants' capacities for statistical language acquisition is the Headturn Preference Procedure (HPP), developed by Stanford psychologist Anne Fernald in 1985 to study infants' preferences for prototypical child-directed speech over normal adult speech.Fernald, A. (1985). Four-Month-Old Infants Prefer to Listen to Motherese ". Infant Behavior and Development, 181-195. In the classic HPP paradigm, infants are allowed to freely turn their heads and are seated between two speakers with mounted lights. The light of either the right or left speaker then flashes as that speaker provides some type of audial or linguistic input stimulus to the infant. Reliable orientation to a given side is taken to be an indication of a preference for the input associated with that side's speaker. This paradigm has since become increasingly important in the study of infant speech perception, especially for input at levels higher than syllable chunks, though with some modifications, including using the listening times instead of the side preference as the relevant dependent measure.

Conditioned Headturn Procedure Similar to HPP, the Conditioned Headturn Procedure also makes use of an infant's differential preference for a given side as an indication of a preference for, or more often a familiarity with, the input or speech associated with that side. Used in studies of prosodic boundary markers by Gout et al. (2004) and later by Werker in her classic studies of categorical perception of native-language phonemes, infants are conditioned by some attractive image or display to look in one of two directions every time a certain input is heard, a whole word in Gout's case and a single phonemic syllable in Werker's. After the conditioning, new or more complex input is then presented to the infant, and their ability to detect the earlier target word or distinguish the input of the two trials is observed by whether they turn their head in expectation of the conditioned display or not.

Anticipatory eye movement While HPP and the Conditioned Headturn Procedure allow for observations of behavioral responses to stimuli and after the fact inferences about what the subject's expectations must have been to motivate this behavior, the Anticipatory Eye Movement paradigm allows researchers to directly observe a subject's expectations before the event occurs. By tracking subjects' eye movements researchers have been able to investigate infant decision-making and the ways in which infants encode and act on probabilistic knowledge to make predictions about their environments.Davis, S. J., Newport, E. L., & Aslin, R. N. (2009). Probability-matching in 10-month-old infants. Methods. This paradigm also offers the advantage of comparing differences in eye movement behavior across a wider range of ages than others.

Artificial languages Artificial languages, that is, small-scale languages that typically have an extremely limited vocabulary and simplified grammar rules, are a commonly used paradigm for psycholinguistic researchers. Artificial languages allow researchers to isolate variables of interest and wield a greater degree of control over the input the subject will receive. Unfortunately, the overly simplified nature of these languages and the absence of a number of phenomena common to all human natural languages such as rhythm, pitch changes, and sequential regularities raise questions of external validity for any findings obtained using this paradigm, even after attempts have been made to increase the complexity and richness of the languages used. The artificial language's lack of complexity or decreased complexity fails to account for a child's need to recognize a given syllable in natural language regardless of the sound variability inherent to natural language, though "it is possible that the complexity of natural language actually facilitates learning."Romberg, A. R., & Saffran, J. R. (2010). Statistical learning and language acquisition. John Wiley & Sons, Ltd. WIREs Cognitive Science. http://www.waisman.wisc.edu/infantlearning/Publications_files/Romberg.Saffran.2010.pdf

As such, artificial language experiments are typically conducted to explore what the relevant linguistic variables are, what sources of information infants are able to use and when, and how researchers can go about modeling the learning and acquisition process. Aslin and Newport, for example, have used artificial languages to explore what features of linguistic input make certain patterns salient and easily detectable by infants, allowing them to easily contrast the detection of syllable repetition with that of word-final syllables and make conclusions about the conditions under which either feature is recognized as important.

Audio and audiovisual recordings Statistical learning has been shown to play a large role in language acquisition, but social interaction appears to be a necessary component of learning as well. In one study, infants presented with audio or audiovisual recordings of Mandarin speakers failed to distinguish the phonemes of the language. This implies that simply hearing the sounds is not sufficient for language learning; social interaction cues the infant to take statistics. Particular interactions geared towards infants is known as "child-directed" language because it is more repetitive and associative, which makes it easier to learn. These "child directed" interactions could also be the reason why it is easier to learn a language as a child rather than an adult.

Bilinguals Studies of bilingual infants, such as a study Bijeljac-Babic, et al., on French-learning infants, have offered insight to the role of prosody in language acquisition. The Bijeljac-Babic study found that language dominance influences "sensitivity to prosodic contrasts." Although this was not a study on statistical learning, its findings on prosodic pattern recognition might have implications for statistical learning.

It is possible that the kinds of language experience and knowledge gained through the statistical learning of the first language influences one's acquisition of a second language. Some research points to the possibility that the difficulty of learning a second language may be derived from the structural patterns and language cues that one has already picked up from his or her acquisition of first language. In that sense, the knowledge of and skills to process the first language from statistical acquisition may act as a complicating factor when one tries to learn a new language with different sentence structures, grammatical rules, and speech patterns.

Important findings

Phonetic category learning