SLA

Second Language Acquisition (SLA) is the scientific study of how people learn languages beyond their first. It investigates the cognitive, linguistic, social, and environmental factors that shape the development of a new language system in the learner's mind. The field draws on linguistics, psychology, cognitive science, neuroscience, sociology, and education -- making it fundamentally interdisciplinary.

SLA is distinct from language pedagogy, though the two are deeply connected. SLA asks how and why languages are learned; pedagogy asks how best to teach them. The applied intersection -- instructed second language acquisition (ISLA) -- bridges the gap.

The Central Questions

SLA research organizes around a set of recurring questions:

What does the learner come to know? -- What is the nature of the linguistic system that develops in the learner's mind?
How does the learner acquire this knowledge? -- What cognitive and social mechanisms drive acquisition?
Why are learners different? -- Why do some learners achieve near-native proficiency while others plateau early, given similar exposure?
What role does the first language play? -- How does prior linguistic knowledge help or hinder new language development?
What role does instruction play? -- Does formal teaching make a difference, and if so, how?
What role does age play? -- Is there a critical or sensitive period for language learning? (See Critical Period Hypothesis)

Historical Development

Behaviorism (1940s-1960s)

The field's intellectual roots lie in behaviorist psychology. Language learning was viewed as habit formation through imitation, repetition, and reinforcement. Errors were seen as interference from L1 habits -- a framework known as Contrastive Analysis. The audiolingual method was its pedagogical expression. By the 1960s, Chomsky's critique of behaviorism and the poverty-of-the-stimulus argument undermined the theoretical foundations.

The Birth of SLA as a Field (1967-1972)

Two publications mark the emergence of modern SLA research:

Corder (1967) -- The Significance of Learners' Errors argued that learner errors are not random failures but systematic evidence of an internal learning process. Errors became data, not defects.
Selinker (1972) -- Introduced the concept of Interlanguage, the idea that learners construct a dynamic linguistic system that is neither their L1 nor the target language but a system with its own internal logic.

Nativism and Universal Grammar (1970s-1990s)

Influenced by Chomsky, nativist approaches asked whether the same innate language faculty (Universal Grammar) that enables first language acquisition also constrains second language development. Research explored whether L2 learners have access to UG, partial access, or no access at all. This tradition generated rigorous formal research but struggled to account for the variability and social dimensions of L2 learning.

The Cognitive-Interactionist Turn (1980s-1990s)

Several major hypotheses emerged that shifted attention from innate structure to the mechanisms of learning:

Stephen Krashen's Monitor Model and Comprehensible Input (i+1) -- input as the primary driver of acquisition
Michael Long's Interaction Hypothesis -- negotiation of meaning during interaction as the mechanism that makes input useful
Merrill Swain's Output Hypothesis -- producing language forces syntactic processing that input alone does not
Richard Schmidt's Noticing Hypothesis -- conscious attention to form is necessary for intake

This era also saw the development of skill acquisition theory, connectionism, and processability theory.

Sociocultural and Usage-Based Approaches (2000s-present)

More recent work has expanded SLA beyond purely cognitive models:

Sociocultural Theory (Vygotsky, Lantolf) -- learning happens through social interaction, mediation, and participation in communities of practice. The Zone of Proximal Development applies to L2 as well as L1 development.
Usage-based approaches (Tomasello, Ellis) -- language is learned through frequency, salience, and pattern extraction from input. There is no innate grammar module; linguistic knowledge emerges from use.
Complex dynamic systems theory -- L2 development is nonlinear, variable, and shaped by the interaction of multiple factors over time. No single variable (input, output, motivation, aptitude) explains acquisition in isolation.

Key Constructs

SLA has produced a set of foundational constructs that appear across theories:

Construct	What It Describes
Interlanguage	The learner's evolving, systematic language system
Fossilization	The cessation of development despite continued exposure
Language Transfer	Influence of L1 structures on L2 production and comprehension
Comprehensible Input	Input understood by the learner despite containing new structures (Krashen's i+1)
Noticing Hypothesis	The claim that conscious attention to form is necessary for acquisition
Output Hypothesis	The claim that producing language pushes learners to process syntax, not just meaning
Interaction Hypothesis	The claim that meaning negotiation during interaction drives acquisition
Affective Filter	Emotional barriers (anxiety, low motivation) that impede acquisition
Input Processing	How learners derive intake from input (VanPatten)

SLA and Language Teaching

SLA research informs teaching practice but does not dictate it. Key findings with pedagogical implications include:

Input is necessary but not sufficient. Learners need massive exposure to the target language, but exposure alone does not guarantee accuracy (the Canadian immersion evidence).
Output and interaction matter. Learners need opportunities to produce language and negotiate meaning, not just receive input.
Explicit instruction has a role. Research consistently shows that some focus on form -- especially at the point of communicative need -- accelerates acquisition compared to purely meaning-focused instruction.
Errors are developmental. Many errors reflect predictable stages of Interlanguage development, not teaching failure. Correction is most effective when timed to the learner's readiness.
Individual differences are real. Age, aptitude, motivation, L1 background, and learning context all shape outcomes. No single method works for all learners.

The gap between SLA research and classroom practice remains a persistent concern. Researchers often study controlled variables in laboratory settings; teachers work with whole humans in messy classrooms. Bridging this gap is the ongoing work of ISLA.