The current study aims to stimulate further interest in interdisciplinary studies of variation in the second-language sound system, to outline the issues that must be considered to establish respect across the various disciplines implicated in this line of research, and to present findings from original research that constitutes a first step toward examining this area of second-language acquisition. By examining the acquisition of two phonetic variants in second-language Spanish, one that is largely geographic (/θ/) and one that is conditioned by a range of social factors (/s/-weakening), we show that a large participant pool is needed to study the acquisition of second-dialect features and that the frequency and graphemic contexts in which learners use these features vary. We also identify a range of individual characteristics that appear to influence the acquisition of a second dialect and show that explaining why some learners use socially-conditioned phonetic features requires a complex account of learning experience that includes but is not limited to length, location and purpose of experience abroad, present patterns of contact with native speakers of the language and psycho-social factors linked to language attitudes.