Learners sacrifice robust communication as a result of a social bias

Languages are subject to many competing pressures, which originate in individual-level learning and communication biases and in social biases reflecting community-level dynamics. Recent work suggests that certain aspects of language structure, such as the cross-linguistic trade-off between case and constituent-order flexibility, originate in learners' biases for efficient communication: Learners drop redundant case but retain informative case in production. Social biases can lead to retention of redundant case, resulting in systems that require more effort to produce. It is not clear, however, whether social biases can influence the use of informative cues. We tested this by exposing participants to a language with uninformative constituent order and two dialects, only one of which employed case. We manipulated the presence of social biases for and against the case dialect. Learners biased towards the no-case dialect dropped informative case without compensating for the resulting message uncertainty. Case was retained in all other conditions.