Spontaneous phonetic imitation—the phenomenon where interacting talkers come to be more similar-sounding—may be an important mechanism in dialect convergence and historical sound change. Recent research has been concerned with whether spontaneous imitation is an automatic (and hence unavoidable) process, or whether it is consciously mediated by social factors (e.g., Giles and Coupland 1991, Goldinger 1998, Pickering and Garrod 2004, Pardo 2006). Recently, Nielsen (2008) suggests that imitation of VOT can be influenced by abstract linguistic knowledge as well. This dissertation presents the result from a project that investigated phonetic imitation of vowels. The results show that talkers accommodate on the first and second formants of the model talker in the task, but that not all vowels are imitated to a significant degree. In this study of American English, only the low vowels /a/ and /æ/ exhibit strong imitation effects, and this effect lies primarily within the F1 dimension. I argue that this is due to the increased repertoire of production variants talkers store for low vowels as a result of the difference in jaw height in accented and unaccented environments. In addition to these findings of phonetic selectivity, the degree to which vowels were imitated were subtly affected by implicit social measures of how the participant felt about the model talker in the experiment. The results of this study suggest that participants only make use of pre-existing tokens within their phonetic repertoire in a shadowing task and that the use of those variants is mediated by implicit social factors. Such results demonstrate that phonetic imitation is not automatic in terms of occurring all the time, but indeed automatic in terms of happening subconsciously. That is, the social factors that mediate the imitation process are not explicit social choices, but implicit socio-cognitive biases.