As explanation of between-speaker differences in speech production moves beyond sex-and age-related differences in physiology, discussion has focused on individual vocal tract morphology. While it is interesting to relate, say, variable recruitment of the jaw to extent of palate doming, there is a substantial residue of arbitrary differences that constitute the speaker's "style". Style differences observed across a well-defined social group indicate group membership. Other style differences are idiosyncratic "habits" of articulation, individual solutions to the many-to-many mapping between motoric and acoustic representations and to the many different attentional trading relationships that can exploit the typical patterns of redundant variation in independent acoustic correlates of any minimal contrast. Perceptual studies of social style differences suggest that perceptibility depends upon the task and upon the hearer's own group membership. The few studies of idiosyncratic differences suggest that speakers perceive each others' productions in terms of their own habits. Thus, perceptual compensation for speaker differences must go beyond mere vocal tract normalization. A promising route for describing how listeners compensate for the arbitrary variation of style is an instance-based (or exemplar) model of speech perception in which the distribution of exemplars is heavily weighted by instances of the speaker's own productions
