It has been proposed that there are cognitive biases in language learning that favour certain patterns over others. This study examines the effects of such bias factors on the learning of the phonology of proper nouns. I take up the phenomenon of compound voicing in Japanese surnames. The results of two judgment experiments show that, while Japanese speakers replicate various kinds of statistical regularities in existing names, they tend to extend only phonologically motivated patterns to novel names. This suggests that phonological naturalness plays a role even in the learning of a highly faithful category of words, namely proper nouns, and provides evidence for the relevance of learning biases in synchronic grammar.
