As one of the first generative acquisition researchers who looked into the competence of heritage speaker bilingual individuals, Montrul’s body of work is rightfully the standard for formal linguistic studies on Hispanic heritage speaker bilingualism. This book is the capstone to a pioneering and novel research program, initiated close to a decade ago and continuing to the present and beyond. Called by different labels across the world (e.g., background language, home language speakers), heritage language bilinguals are child naturalistic acquirers of a minority language spoken at home along with a majority language acquired either simultaneously or subsequently at a very early age. Crucially, however, a heritage language is not the language of the general society in which the child grows up and in which the child is formally educated. Spanish in the context of North America, the language most directly examined in this book, is a clear case of a heritage language; however, any minority language acquired at home that comes in contact with a majority language in any location qualifies as a heritage language.

This volume is innovative in its completeness and the breadth of issues addressed. It successfully demonstrates how and why the investigation of child heritage language bilingual competence outcomes in adulthood adds to many key epistemological discussions pertinent to adult SLA theory, linguistic theory in general, and first language (L1) attrition studies. Furthermore, this work provides a thorough review of an impressive amount of available literature on nonnative ultimate attainment. Montrul shows that age of acquisition is deterministic not only for adult SLA but also for what properties are vulnerable to incomplete acquisition in childhood bilingualism and L1 attrition.

Since the dawn of SLA theories, there has been an active debate as to what outcomes can be reasonably expected in second language (L2) ultimate attainment. Although conceived of differentially by individual researchers and various cognitive-SLA paradigms, implicit to any strong application of the critical period hypothesis to adult language acquisition is the assumption that age of exposure is an inevitably deterministic variable to ultimate native success. Details aside, many proponents of critical or sensitive period explanations of adult L2 and native control competence differences maintain that increasing age of exposure delimits the mere possibility of nativeness. Although not always explicitly stated, it is fair to claim that many of these researchers take the position that language acquired before the onset of a critical or sensitive period should have no other recourse than to be acquired completely. Although Montrul shows that age of acquisition is related to nonnative outcomes, she demonstrates how nonnative outcomes are not limited to adult language acquisition but also obtain even when the language was natively acquired in childhood. She postulates that nonnative outcomes in heritage language bilingualism occur on a continuum of various degrees of incomplete acquisition (i.e., arrested development of the native grammar due to the influence of the majority language, the L2), L1 attrition, or a combination of both. It is significant that Montrul demonstrates that heritage speaker bilinguals of Spanish differ from native controls in many domains of grammar (e.g., use of subject pronouns, differential object marking, aspect, and mood) despite the fact that they have acquired Spanish as a L1 in childhood. This effectively deconstructs the notion that prepubescent language acquisition inevitably results in nativelike convergence, which, in turn, questions the position that child and adult acquisition must be fundamentally different. After all, if child naturalistic acquisition can result in comparative differences from the established monolingual norms without raising questions about the extent to which these learners had accessibility to inborn linguistic mechanisms, then why should the presence of L1-adult L2 differences necessarily mean a fundamental difference? Assuming that quality and quantity of input, language contact, or language transfer as well as (some) affective factors come together to explain heritage speaker outcome differences, it is reasonable to argue that these factors also pertain to the outcome differences noted in adult SLA without the processes and available mechanisms that constrain language learning as being inevitably unique. That heritage speakers empirically test similarly to adult L2 learners for some properties, are outperformed by advanced L2 learners, and are more comparable to native-speaker controls than L2 learners in yet other domains (or subproperties of specific domains) suggest that heritage speakers are different—understanding these differences more completely is the object of this book and much future research. Montrul highlights many issues that must be taken into account when assessing the competence of bilinguals, child and adult alike, and that so-called incompleteness is both relative and dynamic. As such, this book is compulsory reading for all students and researchers who are interested in SLA, bilingualism, and L1 attrition.