The role of linguistic input through a speaker's lifespan

The processes by which a first language is acquired are well understood after many years of
intensive research. Moreover, considerations of language acquisition and learnability have heavily
influenced theoretical models of the architecture of the human language faculty and formal analyses
of the (adult) grammar of different languages. For any language, the full set of its grammatical
properties must be inferred from an 'impoverished' (variable, incomplete) language input present in
the child's environment from birth, guided by a core of universal language properties. E: once the
child acquires an adult-grammar the process is assumed to stop. That may seem plausible in the case
of monolingual children who stay in the places where they were born, however for many other
speakers this is not the case. For instance, speakers who have acquired a complete adult grammar
and are later exposed to another language (e.g. as a result of migration) often report difficulties
speaking their first language: they forget words, expressions and even show grammatical differences
from their monolingual speakers. This phenomenon is known as "first language attrition".
Currently our field lacks a linguistically-based model of first language attrition. We know that
changes to the input a speaker receives may result in restructuring of some properties of the first
language grammar, but it remains unknown what quantitative and qualitative differences in the first
language and second language inputs engender this attrition. For example, is attrition more likely in
grammatical areas where the two languages in question are more similar, or more different? This
requires investigation with different pairs of languages with specific input differences and different
grammatical structures. This work thus necessitates interdisciplinary research integrating language
acquisition theory with two areas of formal linguistic theory: firstly, a comparative, formal
characterisation of the nature of grammatical differences between language pairs for those areas of
the grammar which may exhibit attrition; secondly, in establishing the compatibility of the first
language attrition data with the predictions of current theoretical models of the architecture of the
grammar.

What is the role of input after a first language has been acquired? Does sensitivity to input
still occur after a grammar has been acquired?
- How do speakers perceive-and how are they affected by-changes in the input they
receive through their lifespan?
- What kind of quantitative and qualitative changes in the input lead to first language attrition
and what aspects of an adult grammar are potentially subject to attrition?
- How can theoretical models of the grammar be informed by language attrition data?