Why do languages share structural properties? The functionalist tradition has argued that languages have evolved to suit the needs of their users. By what means functional pressures may come to shape grammar over time, however, remains unknown. Functional pressures could affect adults' production; or they could operate during language learning. To date, these possibilities have remained largely untested. We explore the latter possibility, that functional pressures operate during language acquisition. In an artificial language learning experiment we investigate the trade-off between word order and case. Flexible word order languages are potentially ambiguous if no case-marking (or other cues) are employed to identify the doer of the action. We explore whether language learners are biased against uncertainty in the mapping of form and meaning, showing a tendency to make word order a stronger cue to the intended meaning in no-case languages.