Behaviorist theory for second language acquisition is founded on the idea that when learning language children imitate the people around them and learn through positive reinforcement (Broad, 2020). The environment is the key source of learning; language learners learn through hearing the language and reinforcement of correct usage. This theory, founded on the work of B. F. Skinner, was influential from the 1940s to 1970s and underpinned approaches to language teaching founded on imitation and practice.

From this perspective habits from a learner’s first language (L1) could interfere with the required new habits for the second language (L2); errors were therefore related to transfer from the first language (Lightbrown & Spada, 2021). It has been found that errors in L2 cannot always be explained in relation to transfer from L1; as a result the behaviorist perspective is inadequate for understanding language acquisition (Lightbrown & Spada, 2021). The behaviorist theory for second language acquisition frames different languages as separate and does not account for the connection interplay between languages in relation to how multilingual learners make meaning.

The innatist perspective on language acquisition is founded on the idea that there are common principles underpinning language and language learning is an innate ability. The focus is on biological dimensions of language acquisition rather than social, functional, and multilingual aspects. In contrast to behaviorist theory, from an innatist perspective the environment only plays a basic role in language acquisition (Lightbrown & Spada, 2021). This theoretical perspective is critiqued for ignoring the social dimension of language acquisition and implementing a monolingual worldview (Sridhar & Sridhar, 2018). An example of this perspective is Chomsky’s universal grammar hypothesis of a set of principles common to all languages (Broad, 2020). While Chomsky was not focusing on SLA his theory has had a strong influence on this field.

Krashen’s Monitor Model is a well-known model of SLA that was influenced by Chomsky’s theory of L1 acquisition. Krashen’s model is founded on five hypotheses (Lightbrown & Spada, 2021). 

1. Acquisition/learning hypothesis: there is an interplay between acquisition, exposure to L2, and learning, conscious form and rule learning, with more language acquired than learned.
2. Monitor hypothesis: learners use the rules they have learned to monitor and adjust their communication in L2.
3. Natural order hypothesis: L2 learning occurs in a predictable sequence.
4. Comprehensible input hypothesis: L2 acquisition requires exposure to language that is comprehensible and also contains language that is one step beyond their current level.
5. Affective filter hypothesis: the affective filter accounts for feelings and anxiety that may result in poor learning outcomes.

Research and theories from cognitive psychology have informed models of SLA since the 1990s. Cognitive and developmental psychologists argue that “L1 and L2 acquisition draw on the same processes of perception, memory, categorization and generalization” (Lightbrown & Spada, 2021, p.119). From this perspective the difference is related to the circumstances surrounding learning and the learner’s prior knowledge of language. Theories of language learning from a cognitive perspective draw on broader learning theories, highlighting the learner’s ability to process and learn through experience. This perspective considers language as just one of many complex knowledge systems (Lightbrown & Spada, 2021). In a similar way to critiques of the innatist perspective, cognitive theories of SLA are critiqued for limited consideration of how social and cultural interaction, and emotional variables influence language development.

Sociocultural theories of language acquisition consider innate learning ability and environmental/social factors (Lightbrown & Spada, 2021). Vygotsky, while not focusing on language acquisition was influential in this area (Pathan et al., 2018). Vygotsky perceived thought as internalised speech, and with social interaction this internalised process emerged as language (Lightbrown & Spada, 2021). Learners advance their language learning through supportive social structures and imitation experiences. There is a focus on the relationship between language acquisition and the learner’s experience and cognitive development. Developing this further, communicative theory of second language emphasises the communication of meaning in interaction, drawing on knowledge of structures and the function of language (Broad, 2020).