ABSTRACT
As a field, Second Language Acquisition (SLA) is vibrant. It began in the late 1960s, with key initial developments during the decade of the 1970s. Most scholars agree that the coming of age as an autonomous discipline happened some time at the end of the 20th century, after less than 40 years of exponential growth. Since then, a prodigious expansion in research and theorizing has occurred that continues unabated at the time of this writing. The field of SLA is also decidedly interdisciplinary, both in its origins and its development, a quality that is felt in the epistemological diversity of its theories as well. SLA interconnects with four neighboring fields, some of them also relative newcomers in academia: language teaching, linguistics, child language acquisition, and psychology. In more recent years, it has also developed ties with other disciplines, notably bilingualism, cognitive science, education, anthropology, and sociology. Given this vibrant disciplinary landscape, the second decade of the 21st century is an opportune time to reflect on the theories in SLA that offer the most viable explanations about humans’ capacity to learn additional languages (henceforth L2) later in life, after having learnedfrom birth to roughly age four-the first language (in the case of a monolingual upbringing) or languages (in the case of a bi/multilingual upbringing).
