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, Carmen Muñoz Lahoz (dir. tes.) , Elsa Tragant Mestres de la Torre (secret.) , David Singleton (voc.) , Maria González-Davies (voc.) , Rosa María Jiménez Catalán (voc.) This dissertation analyses the effects of age on the acquisition of vocabulary in English as a Foreign Language (FL). It focusses on productive vocabulary because it is a crucial aspect in language learning which is also vital in communication, both oral and written, and it has often been the neglected component in research on age. Studies on age in naturalistic contexts have usually shown that 'the earlier one starts learning the language, the better'. Although in the short term Early Starters (ES) are outperformed by Late Starters (LS), due mainly to the superior cognitive maturity and the quickest rate of development of the latter, in the long run ES will catch up with LS and will eventually overtake them. Our educational system also promotes an early start to the FL, as the age at which English is first introduced at schools has progressively been brought down in the recent years. However, results in instructional settings are not as clear-cut as the ones obtained in natural contexts and 'the earlier the better' assumption does not always seem to apply in a straightforward way. The present work analyses the oral and written production of two groups of Catalan/Spanish bilinguals who learn English at school as a FL. One of the main aims of the research is to determine if, in the long term, towards the end of secondary education, there are consistent differences between a group of ES (who started at 8) and one of LS (who started at 11) as regards productive vocabulary. The design presented also allows to study the development and draw comparisons between the groups from primary education onwards (data collections were carried out after 200, 416, 726 and 800 hours of exposure). Therefore, the analyses are performed by controlling the Amount of Exposure that learners have received, their Cognitive Maturity (Age at Testing) and their Age of Onset (Starting Age). In order to analyse the participants' production, both intrinsic and extrinsic vocabulary measures are used. It is also a research purpose of the present work to survey some of the widely-used measures of lexical richness and to evaluate new ones, such as D and P_Lex. In addition, as the Second Language Acquisition research field is very much in need of testing tools, especially as regards vocabulary, this thesis presents one of the first methods to compute estimates of productive vocabulary size for different tasks. The process to carry out the estimations has been automated into V_Size, a new computer tool.Results show that in the long term, ES do not surpass LS in a formal context as far as lexical production is concerned. The same takes place even if ES receive some more exposure. This LS advantage seems to be present since the first stages of learning the FL, even though both groups show a boost in vocabulary from the age of 13 onwards. The findings are discussed in the light of other results found in natural and formal contexts for lexis and other language components. They are also interpreted in relation to other factors that may contribute to a successful learning apart from an early start. Results on different lexical measures are considered for research in the field and potential advantages of the new V_Size are described. Therefore, not only does the thesis bring new evidence to research on age and FL acquisition, but it also offers new insights into productive vocabulary testing.
