This chapter presents an overview of what is currently known about phonetic and phonological first language (L1) attrition and drift in bilingual speech and introduces a new theory of bilingual speech, Attrition & Drift in Access, Production, and Perception Theory (ADAPPT). Attrition and drift are defined and differentiated along several dimensions, including duration of change, source in second language (L2) experience, consciousness, agency, and scope. We address why findings of attrition and drift are important for our overall understanding of bilingual speech and draw links between ADAPPT and well-known theories of L2 speech, such as the revised Speech Learning Model (SLM-r), the Perceptual Assimilation Model-L2 (PAM-L2), and the Second Language Linguistic Perception model (L2LP). The significance of findings revealing attrition and drift is discussed in relation to different linguistic subfields. The chapter raises the question of how attrition and drift potentially interact to influence speech production and perception in the bilingual’s L1 over the life span; additional directions for future research are pointed out as well.

Chapter 6 moves from the focus of Chapter 5 on macro level social categories to examine approaches to the study of within-individual variation and the construction of linguistic style and social identity.In particular, the chapter turns to what sociophonetic research, given its ability to examine gradient phonetic shifts, brings to this pursuit.The chapter begins by considering the study of individual variation in linguistics and phonetics more generally and then moves to discuss early approaches to sociolinguistic style and how those approaches evolved to present day interests in language and identity, style and interaction, within the realm of what is referred to as "third wave" research.Focusing in on such third wave approaches, the chapter then examines how sociophonetic studies on sibilant variation have provided an opportunity to explore both the social and cognitive aspects of socio-indexical variation.Finally, the chapter turns to what sociophonetic research, given its ability to hone in on very particular aspects of phonetic features, can bring to the study of linguistic performance, attending to studies that examine sibilant variation and speech perception.