This thesis constitutes a mixed-methods enquiry into how vocabulary develops across adolescence, within the context of New Zealand secondary schools. A quantitative approach was adopted to investigate vocabulary use in authentic written essays produced by secondary school English students (N=141) belonging to three age groups: 13-14, 15-16, and 17-18, from eight schools. Essays were analysed for the following three lexical richness features: lexical variation, lexical sophistication, and lexical density. With links between these lexical richness features and vocabulary size/skill in vocabulary use (Vermeer, 2000; Ravid & Zilberbuch, 2003; Malvern, Richards, Chipere, & Durán, 2009), signs of development were studied through comparison of scores across the three age groups. Quantitative findings indicate significant lexical development across year levels in the data set. Furthermore, the findings suggest that within the period of adolescence there is an even more specific period in which substantial development takes place: 15-18 years, or later adolescence.The qualitative aspect of this study focussed on identifying teacher perspectives on influences from within the secondary school context impacting on vocabulary development during this significant period of acquisition. Seven secondary school English teachers were interviewed on the subject of lexical development as it occurs within the schooling environment. Contributions from the school curriculum to vocabulary acquisition were observed, with spikes in curriculum difficulty from year 11 (age 15-16) onward corresponding with the developmental spike observed in the quantitative data further supporting this observation. Non-schooling related influences were also identified, including cognitive development, reading habits, and attitude and orientation toward vocabulary.The present study contributes to the growing field of later language acquisition through identification of a possible period of heightened development within the adolescent years. Importantly, it also highlights factors in students’ everyday school lives which may contribute to their lexical development, raising implications both for those wishing to promote lexical development within the secondary school population, and more globally for our understanding of how heightened development occurs during this period. The study concludes with implications for theory, research and practice, together with limitations of the study and future research directions arising from this research.
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