Language attrition occurs when people lose fluency in their native language as a result of becoming bilingual or multilingual. The process of acquiring new languages can affect a person's usage of the language that they were born into as well as those used later in life. With international immigration becoming much more common in the 20th century, the field of applied linguistics has created models to better understand how acquisition of new languages results in language attrition. Loss of language skills can be the result of many different factors and can can ultimately lead to what linguists term "language death."
Linguists use the term "first language attrition" to describe the gradual loss of a first language (L1) as the migrant gains proficiency in a second language (L2). It has been observed that language attrition works in both directions. Native speakers' L1 skills can undergo changes in fluency while they acquire L2 skills. The extent to which L1 is impacted can be correlated with the degree to which L2 becomes dominant in the person's life, combined with diminishing exposure to L1 and its surrounding culture. Linguists have tried to identify the degree to which interference between L1 and L2 can be considered normal versus abnormal, but, without a standard of language "normalcy," current thinking tends to see language attrition as a continuum rather than a series of fixed events.
Research has shown that both first language acquisition (FLA) and second language acquisition (SLA) are impacted by external factors, such as the degree of language exposure, as well as the person's language aptitude and motivation. These external factors tend to affect L2 acquisition more so than L1. Both those undergoing L1 attrition and L2 learners often use the language in ways that differ from native speakers, especially in the areas of grammar and syntax. These changes appear to be the result of incompatibilities between the two language systems rather than a change in the speaker's underlying linguistic skills and understanding.
The process of language attrition is still a theoretical field of study. Some of the factors linguists continue to investigate include the regression hypothesis, which holds that L2 loss occurs more quickly than that of L1 due to psychological as well as social factors. The age at which one acquires his or her L1 and L2 skills can influence how quickly either may be subject to attrition. Studies of pre- and post-puberty migrants indicate that prepubescent language learners tend to lose their L1 skills more slowly while acquiring fluency in L2 more quickly.