|Abstract:||Within the framework of Optimality Theory (Prince and Smolensky 2004), Steriade (2001a, b) proposes the P-map hypothesis, whose fundamental tenet is that the rankings of faithfulness constraints are grounded in perceptual similarity rankings. This article provides empirical support for this hypothesis. In Japanese loanword phonology, only a voiced geminate, but not a singleton, devoices to dissimilate from another voiced obstruent within a single stem. Based on this observation, I argue that the [+voice] feature is protected by two different faithfulness constraints, Ident(+voi)Sing and Ident(+voi)Gem, and they are ranked as Ident(+voi)Sing >> Ident(+voi)Gem in Japanese. I further argue that this ranking is grounded in the relative perceptibility of [+voice] in singletons and geminates, and this claim is experimentally supported. Overall, this paper has a general theoretical implication that phonetic perceptibility can directly influence patterns in a phonological grammar.