Heritage language acquisition theories are highly contested. The most common theory is the Incomplete Acquisition Theory, Therefore, heritage language learners, who typically switch to the dominant culture's language when they enter school and, consequently, experience a decline in input in the heritage language, do not completely acquire their first language. As heritage language learners are rarely schooled in their L1, they are often orally proficient but illiterate or have underdeveloped written comprehension and production in the heritage language. In treating heritage language learning as interrupted L1 acquisition, in which the learner has access to
Universal Grammar, it is expected that heritage language learners have knowledge of concepts found in early stages of L1 development, such as binding constraints,
wh-movement, and aspects of
lexical semantics, and struggle with grammatical concepts that require sustained exposure and practice in school-aged children, such as specialized vocabulary and uses of the subjunctive in Spanish.
Attrition An incomplete grammar may be the result of attrition and fossilization of concepts in the L1 due to insufficient input once the child has switched to the dominant language. Attrition is the loss of, or failure to make full use of, "grammatical knowledge previously acquired". Fossilization occurs when a speaker lets go of non-core grammatical concepts but retains the basic core structure of the language. Thus, this theory must be considered in conjunction with theories on input variation.
Input Theories of acquisition involving variations of input postulate that heritage language learners' production of the heritage language diverges from that of their monolingual peers who speak the same language because the two groups are exposed to different dialects and quantities of input.
Dialect variation Theories surrounding dialect variation suggest that errors or deviations from the standard dialect made by a heritage language learner may reflect the acquisition of a non-standard variety or informal register of the heritage language, which include variations on certain properties or the lack of certain properties found in the standard dialect. Heritage language learners are often only exposed to one dialect or colloquial variety of the heritage language, unlike their monolingual peers who interact with a standard monolingual dialect found in formal instruction. Hours of input may be particularly restricted once the heritage speaker switches to the dominant language.
Cross-linguistic influence Cross-linguistic influence may contribute to heritage speakers' competence divergence. Heritage language learners show a tendency to overuse grammatical properties that are found in both the heritage language and the dominant language. Furthermore, heritage language learners may prefer grammatical structures from the dominant language and transfer them into the heritage language. To learn more about
language transfer in bilingual individuals, see
Crosslinguistic influence. == Identity and heritage language learning ==