The modem era of developmental psycholinguistic research has been marked by a debate over the variable contributions of nature and nurture to the language acquisition process. Although theoretical consensus eludes the field, it is now generally agreed that while nature provides the tendency, nurture provides the opportunity for language to come to humans and the research interest has shifted toward questions about the nature of the nurture.
Understanding the complex synergy that exists between evolutionary priming and environmental triggers, and describing the mechanisms and processes of each, now defines many contemporary acquisition studies. This thesis takes up the case for nurture and considers an environmental determinant of language acquisition: child-directed speech styles and their relationship to emergent vocabulary growth. It is the contention held here that particular types of socio-communicative
mother-child interactions facilitate vocabulary but describing and understanding these facilitative pathways first requires conceptual models that capture the content, scope and dynamism of human speech production. These are currently lacking in the child language literature.
A critical environmental correlate of child language acquisition is the type of language children hear around them and a fundamental question in acquisition research is how best to describe mothers' speech styles. Many studies have been devoted to describing the global attributes of child-directed speech (CDS) and the facilitative role this distinct speech register may play in a child's acquisition of their first language. Research also considers if and how child-directed speech styles vary between individuals and how certain ways of speaking to children may facilitate or inhibit the way they themselves learn to speak. While measuring environmental influences has
proven to be a difficult task, it has come to be understood that speech style differences do exist between mothers, though few functions of language have been consistently proven to be related across studies and across contexts. This might be due to the efficacy of the instruments used to assess speech styles. Two instruments are used here to describe variation of speech styles among mothers and to understand the potential these styles have to affect child language outcomes. The studies here raise questions about the developmental effect that maternal linguistic variation might bring by relating a mother's speech style to the vocabulary outcomes of her toddler. This has important implications for understanding the wide variation that exists in the rate at which children acquire language, and for interactionism and speech act theory.
Many investigations into maternal speech styles can be characterised by their focus on atypical populations, older child
subjects, the use of small sample sizes or a narrow focus on one or a just a few aspects of CDS. To date, there has been a paucity of large-scale corpus Q studies of the impact of speech styles for typically-developing toddlers on the cusp of productive speech. Moreover, existing work has typically viewed features of individual speech styles as monolithic, stylistically discrete traits that remain more or less stable over time and across situations. An alternative paradigm might seek to identify any stylistic variation that might exist between mothers as a result of state changes.
The aim of the studies presented here was to extend the work on maternal speech styles by exploring if style differences influence toddler vocabulary outcomes. An archival analysis of over 300 hours of laboratory-derived transcripts from The New England corpus of the CHILDES database was used to consider the socio-linguistic interactions between 51 English-speaking mother-child
dyads when the children were aged between 14-32 months. This work further contributes to the literature by considering the mediating influence of situational context on variation within personal speech styles as it seeks to determine if interaction exists between characteristic trait features of speech style and those that may occur as a result of changes of state.
This approach was informed by the literatures on Cognitive Socialization and was motivated by dual speech styles hypotheses, both theoretical and methodological: 1). that children who are exposed to linguistically rich CDS will themselves become linguistically nimble. Therefore, the importance of instruments that can capture linguistic diversity in language input are of importance to the study of child language; and 2). although individual speech styles may be globally characterised in terms of enduring trait features, these characteristic ways of performing may be overridden, or altered by changes
of state indicating the need for context to be controlled for in studies of vocabulary development.
From the child language literature, two approaches that characterise mothers' speech styles were selected for analysis: the Interactive model, based on the Interactive Model of Language Intervention (IMLI), espoused by Girolametto and colleagues (1999) and others, and the Inventory of Communicative Acts-Abridged (INCA-A), developed by Ninio and Snow (1996). Models such as these provide the analytical framework for descriptions about the characteristics of mothers' speech styles. However, while the overall theoretical orientation that informs both approaches is interactionist, each model emphasises different aspects of the child-directed speech. This indicates that there exists a fundamental divide between the orientations which inform each model. The objective was to explore these orientations and determine which model better describes maternal speech styles.
The studies presented here identified gaps in terms of how the child language literature currently characterises linguistic style. A significant finding was that establishing correlation comprehensively and conclusively is problematic, as the full complexity inherent in mothers' speech is unable to be captured on the existing instruments. Therefore, before further insight about the nature and affect of maternal speech styles can be gleaned, particular instrument efficacy and study methodology issues need to be addressed. These include instrument issues, such as refining the content coverage of speech styles instruments to be more inclusive of the fuller spectrum of speech style characteristics whilst also factoring out rarely occurring linguistic phenomena; and design issues, such as the inclusion of additional instruments and methods, such as experimental manipulation of the linguistic phenomena of interest. This will improve the possibilities of capturing
the complexity of individual speech style.
The main findings indicated some evidence of casual links between a mother's style and her child's vocabulary outcomes, and some evidence of mothers' speech varying stylistically by socio-interactional context. This departs from studies which have demonstrated stability of speech style over time and context and indicates that although context has often been disregarded as a factor in style analysis studies, it may be useful to conceptualise mothers' speech styles as including a state aspect, in addition to the stylistically discreet trait aspects previously identified in the literature on Linguistic style. Theoretically, these results build the case for the critical role of nurture, in relationship to other correlates, in the language acquisition process. They highlight the ultimate effectiveness of environmental changes to improve the human situation. Further, the results clearly elucidate that understanding the
nature of nurture will be complex indeed.