Some learners of second languages manage to attain near-native levels of competence, whereas others settle for very incomplete levels even after decades of using a second language. This wide variation in adult attainment contrasts sharply with the uniform success that children have in learning their first language, suggesting that there is a critical period for language learning that expires sometime during early adolescence. Accounts for this decline have considered the impact of biological mechanisms such as lateralization, myelination, metabolic decline, synaptic pruning, and changes in NMDA receptor subtype, as well as network features such as entrenchment and gang effects. None these accounts can explain the full range of patterns of success and failure across the areas of phonological, lexicon, syntax, intonation, and conversational pragmatics.
The Unified Competition Model refocuses this discussion in terms of the dynamic interplay between a set of risk factors facing adult learners and a set of protective or support factors that they can use to overcome the barriers established by the risk factors. The risk factors are entrenchment, transfer, overanalysis, and social isolation. The support factors combating these risks are resonance, decoupling, chunking, and participation. The operation of each of these processes can be modeled by collecting data from experimentation and corpus analysis. By examining in detail the differential operation of each of these factors on each linguistic level, we can gain a fuller picture of differences in patterns of second language acquisition in adulthood from which we can formulate effective ways of improving learning success.