The third and final volume in Principles of linguistic change (PLC) may be viewed as the resolution not only of themes explored in the previous two volumes but also of questions that have driven Labov's research agenda over the last fifty years. In Volume 1 (PLC1; Labov Reference Labov1994), he examined the regularity of sound change and reviewed structural and functional explanations. In Volume 2 (PLC2; Labov Reference Labov2001), he looked for the leaders of change in the speech community and proposed models for transmission and incrementation. The focus of Volume 3 (PLC3) is the origins and consequences of the increasing divergence of North American English (NAE) dialects (Labov, Ash, & Boberg Reference Labov, Ash and Boberg2006). Ch. 1, “Cognitive and cultural factors,” sets out the main arguments of the volume and defines the cognitive factors involved in transmitting and decoding information through language, the social factors involved in face-to-face interaction, and the large-scale factors of cultural patterns. Here also the analysis of English vowels (presented in Labov, Yaeger, & Steiner (Reference Labov, Yeager and Steiner1972) (LYS) and PLC1) and the ongoing vowel shifts in NAE are reviewed. Each of the four sections of the volume deals with a different set of interrelated questions.

The consequences of dialectal divergence to the transmission of information are examined in Part A, “The cognitive consequences of linguistic change,” which presents a series of studies of cross-dialectal comprehension that address the question of whether language change contributes to misunderstanding. In Ch. 2, “Natural misunderstandings,” the analysis of a database of natural misunderstandings collected in everyday life suggests that hearers do not access a “pan-dialectal” system but rather filter speech through their own phonemic categories. This question is pursued experimentally with speakers and judges from cities undergoing different shifts in Ch. 3, “A controlled experiment on vowel identification,” and Ch. 4, “Gating experiments on cross-dialectal comprehension.” Ch. 3 replicates the Peterson–Barney vowel identification experiment, while Ch. 4 presents a series of “gating” experiments (subjects are asked to identify a word, first in isolation and then in phrasal and sentential contexts). Since overall success rates in both studies are relatively low and the advantage of local listeners is limited, sound change seems to impede comprehension not only across but also within dialects.

Part B, “The life history of linguistic change,” considers the origins, driving forces, and ends of language change. Ch. 5, “Triggering events,” examines the historical context of NAE vowel shifts, triggered by movements across subsystems. For example, the low back merger of /o/ and /oh/ triggered both the Canadian Shift and the Pittsburgh Shift, and the Northern Cities Shift (NCS) was triggered by the raising of /æ/, which itself arose from the development of a dialect koiné along the trade route of the Erie Canal. The principles that govern mergers and chain shifts (proposed in LYS and PLC1) are reviewed in Ch. 6, “Governing principles.” The irreversibility of mergers may be overturned through the absorption of small-city dialects by regional norms. Since chain shifts act to preserve distinctions obscured by mergers, drag chains are more frequent than push chains. Ch. 7, “Forks in the road,” focuses on points in time when an unstable linguistic situation may be resolved in more than one way. When the adoption of one way triggers further changes, dialect divergence occurs, as detailed in Ch. 8, “Divergence.” For example, different dialects resolved the earlier (un)rounding of /o/ either through the low back merger or through further shifts that increased the distance between /o/ and /oh/. The various social and cultural factors proposed to motivate linguistic change (local identity, gender, communities of practice) are outlined in Ch. 9, “Driving forces.” Gender effects can be interpreted in terms of differences in language transmission between boys and girls. However, the extent and uniformity of the NCS across a vast region suggest that cultural factors go beyond those of local interaction or social identity. These cultural factors are explored in Ch. 10, “Yankee cultural imperialism and the Northern Cities Shift,” which seeks ideological correlates of linguistic behavior through “blue state” political patterns that are traced to the Yankee settlement of the Inland North. Ch. 11, “Social evaluations of the Northern Cities Shift,” explores ideological correlates experimentally by asking listeners from different states to infer speakers' political and cultural stances on the basis of speech extracts from different dialect regions. Thus, ideology is both a driving force behind change and a barrier to its expansion. Ch. 12, “Endpoints,” accounts for completed changes.

The cognitive basis of phonemic distinctions is the focus of Part C, “The unit of linguistic change.” Ch. 13, “Words floating on the surface of sound change,” examines the regularity of sound change and what its fundamental unit is. Lack of evidence for lexical diffusion in NAE vowel shifts lends support to the Neogrammarian view that language change targets phonemes rather than words, although residual lexical effects suggest that some words participate differently in ongoing change. Ch. 14, “The binding force in segmental phonology,” adduces the absence of allophonic chain shifting as evidence for a binding force that counters the disruptive effect of co-articulation on phonemic unity.

Two different types of language change are distinguished in Part D, “Transmission and diffusion.” Transmission, the unbroken sequence of native-language acquisition by children, is reflected in the family-tree model of change. Imperfect transmission results in internal language change (“change from below”), which advances through generational incrementation. Diffusion, which is reflected in the wave model of language change, results from language contact among adults, who replicate detailed features of language structure poorly. To demonstrate the difference between the two types, Ch. 15, “Transmission and diffusion,” provides evidence from geographically separated communities. Diffusion across groups within the same community is illustrated in Ch. 16, “Diffusion across communal groups”. While majority ethnic groups (German, Italian, Scandinavian, Polish) assimilated to the linguistic mainstream, groups isolated through segregation developed other norms. For example, although African American English (AAE) dialects show some participation in changes occurring in the surrounding white vernacular, phonetic conditioning is weakened. Latino groups shift to the local white vernacular or towards AAE or preserve features of Spanish influence, depending on individual differences in social identification.

In Ch. 17, “Conclusion,” Labov considers the relation between cognitive and cultural factors. The tendency of vowel shifts to maximize dispersion may result from long-term “repair strategies” that address the effects of (internal or external) disturbances on vowel subsystems. Drawing on analogies between human language and animal communication systems, he suggests that humans inherited two types of intelligence—social and representational—and that language change may result from social-intelligence factors that are orthogonal to, or even antagonistic toward, the representational function of language.

The evident futility of searching for a single, universal account of language change is reflected through the use of the plural “principles” in PLC's title. Change is driven both by representational factors (cognitive) and by interactional and ideological factors (social or cultural). While the former might be considered universal (a human evolutionary inheritance), the latter are clearly contingent. Labov relates linguistic variation to genetic mutation by raising the “Darwinian paradox,” in which linguistic evolution resembles biological evolution but lacks the fundamental mechanism of natural selection (selection by environmental factors). However, while natural selection may not be operative in linguistic “evolution,” linguistic variation may be subject to types of selection that operate in other areas of cultural change (e.g. Dawkins Reference Dawkins1976; Durham Reference Durham1991). Thus, PLC3 begins to suggest a possible convergence of the study of language variation and change with other work on human cognitive evolution and its relation to language (e.g. Christiansen & Kirby Reference Christiansen and Kirby2003). Labov's distinction between social factors (local, interactional) and cultural factors (large-scale, ideological) raises a challenge for research in sociolinguistic variation and change, which normally does not make such a distinction.

In another sense, however, the title of PLC is a bit of a misnomer—these principles account for one type of change (vowel shifts) in a geographically defined set of dialects (North America) of one language (English). Although Labov cites studies on other varieties of English or on other languages that might be seen to support or refute these principles, it remains to be seen whether they can account for other types of change or to changes occurring in other languages (cf. Trudgill Reference Trudgill2011). For example, situations of grammatical change are less clearly driven by cognitive factors related to maximal dispersion within a system (e.g. Kroch Reference Kroch1989; Poplack & Malvar Reference Poplack and Malvar2007) and seem to be less available to the “sociolinguistic monitor” that mediates the social and cultural factors. In this sense, PLC should not be taken as the final word on language change but as an exemplary framework for further study. As Labov has been telling us for almost fifty years, the best way of studying language change is by “paying close attention to what is going on around us.”