First-language acquisition of morphology refers to the process whereby native speakers gain full and automatic command of the inflectional and derivational machinery of their mother tongue. Despite language diversity, evidence shows that morphological acquisition follows a shared path in development in evolving from semantically and structurally simplex and non-productive to more complex and productive. The emergence and consolidation of the central morphological systems in a language typically take place between the ages of two and six years, while mature command of all systems and subsystems can take up to 10 more years, and is mediated by the consolidation of literacy skills. Morphological learning in both inflection and derivation is always interwoven with lexical growth, and derivational acquisition is highly dependent on the development of a large and coherent lexicon. Three critical factors platform the acquisition of morphology. One factor is the input patterns in the ambient language, including various types of frequency. Input provides the context for children to pay attention to morphological markers as meaningful cues to caregivers’ intentions in interactive sociopragmatic settings of joint attention. A second factor is language typology, given that languages differ in the amount of word-internal information they package in words. The “typological impact” in morphology directs children to the ways pertinent conceptual and structural information is encoded in morphological structures. It is thus responsible for great differences among languages in the timing and pace of learning morphological categories such as passive verbs. Finally, development itself is a central mechanism that drives morphological acquisition from emergence to productivity in three senses: as the filtering device that enables the break into the morphological system, in providing the span of time necessary for the consolidation of morphological systems in children, and in hosting the cognitive changes that usher in mature morphological systems in both speech and writing in adolescents and adults.
Empirical and theoretical research on language has recently experienced a period of extensive growth. Unfortunately, however, in the case of the Japanese language, far fewer studies—particularly those written in English—have been presented on adult second language (L2) learners and bilingual children. As the field develops, it is increasingly important to integrate theoretical concepts and empirical research findings in second language acquisition (SLA) of Japanese, so that the concepts and research can be eventually applied to educational practice. This article attempts to: (a) address at least some of the gaps currently existing in the literature, (b) deal with important topics to the extent possible, and (c) discuss various problems with regard to adult learners of Japanese as an L2 and English–Japanese bilingual children. Specifically, the article first examines the characteristics of the Japanese language. Tracing the history of SLA studies, this article then deliberately touches on a wide spectrum of domains of linguistic knowledge (e.g., phonology and phonetics, morphology, lexicon, semantics, syntax, discourse), context of language use (e.g., interactive conversation, narrative), research orientations (e.g., formal linguistics, psycholinguistics, social psychology, sociolinguistics), and age groups (e.g., children, adults). Finally, by connecting past SLA research findings in English and recent/present concerns in Japanese as SLA with a focus on the past 10 years including corpus linguistics, this article provides the reader with an overview of the field of Japanese linguistics and its critical issues.