Substantial empirical research has revealed that temporal and spectral phonetic vowel reduction occurs in "easy" processing contexts relative to "hard" processing contexts, including effects of lexical frequency, lexical neighborhood density, semantic predictability, discourse mention, and speaking style. Theoretical accounts of this phonetic reduction process include listener-oriented approaches, in which the reduction reflects the talker's balancing the comprehension needs of the listener with production effort constraints, talker-oriented approaches, in which reduction is argued to result entirely from constraints on speech production processes, and evolutionary approaches, in which reduction results directly from long-term interactive communication within a community. Recent research in our laboratory has revealed complex interactions among the linguistic, social, and cognitive factors involved in phonetic vowel reduction processes. These interactions reveal variation in the robustness of phonetic reduction effects across linguistic factors, as well as different patterns of interactions among linguistic, social, and cognitive factors across acoustic domains. These interactions challenge aspects of each of the three existing models of phonetic reduction. We therefore propose that a more complex view of the relationship between processing demands and phonetic vowel reduction processes is necessary to account for these observed patterns of variation.