| Summary: | <p>Verbal backchannels – short responses such as 'uh-huh' and 'mhm' given by an interlocutor to the main speaker – have been studied extensively for several decades. The great majority of the research has been descriptive or based on backchannel uses. In contrast, little has been reported of their effects on spoken interaction and almost no research has examined their effects on second language (L2) speech. Given that first language (L1) backchannel norms vary, L2 speakers unaccustomed to different norms could be affected when exposed to such variation. This thesis investigated such effects through the use a quasi-experimental repeated measures design that compared the effects of two backchannel frequencies – one approximately a third of the other – on L2 English speech. The 37 L1 Japanese and 34 L1 Mandarin Chinese participants spoke in English to an interlocutor who varied the frequency of backchannels that they were given in different dyadic interactions. The resultant audio recordings were transcribed and analysed using common measures of speech complexity, accuracy and fluency. Multivariate analyses of variance and t-tests helped show that the fluency of each group was increased when the higher of the two frequencies was given and that, while the accuracy of the Japanese group did not alter, the Chinese group was less accurate in one set of interactions when receiving the higher frequency of backchannels. Effect sizes for these changes (d = 0.19–0.87) were comparable with other studies that used the same measures of fluency and accuracy. There were no statistically significant differences for measures of complexity. The findings show that the contribution of L1 norms to the effects of backchannels on L2 interactions is not as clear-cut as assumed by previous research. The implications of the findings extend into language testing, teaching, theory and research methods.</p>
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