We investigate whether the patterns of phonotactic well-formedness internalized by language learners are direct reflections of the phonological patterns they encounter, or reflect in addition principles of phonological naturalness. We employed the phonotactic learning system of Hayes and Wilson (2008) to search the English lexicon for phonotactic generalizations and found that it learned many constraints that are evidently unnatural, having no typological or phonetic basis. We tested 10 such constraints by obtaining native-speaker ratings of 40 nonce words: 10 violated our unnatural constraints, 10 violated natural constraints assigned comparable weights by the learner, and 20 were control forms. Violations of the natural constraints had a powerful effect on ratings, violations of the unnatural constraints at best a weak one. We assess various hypotheses intended to explain this disparity, and conclude in favor of a learning bias account.