Production efficiency can cause grammatical change: Learners deviate from the input to better balance efficiency against robust message transmission.

The idea that human languages have properties suitable for efficient communication has permeated linguistic theorizing. Indirect correlational support for this idea has come from cross-linguistic synchronic and diachronic data. However, direct causal tests have been lacking. We directly test whether biases operating during language learning can cause learners to deviate from the input they receive towards output languages that better balance production efficiency against robust message transmission. We employ miniature language learning experiments to address this question for a well-documented cross-linguistic correlation between constituent order flexibility and the presence of case marking in a language. Participants were exposed to novel miniature languages that had optional case marking and either fixed or flexible constituent order. Between participants, we manipulated the amount of time and effort associated with the production of case marking. We find that learners introduced the cross-linguistically observed trade-off between case marking and constituent order flexibility into their output languages. Critically, learners only did so when case-marked nouns required additional effort compared to non-case-marked nouns. Thus, the present study suggests that even abstract grammatical properties of languages can be shaped by a balance between production efficiency and robust message transmission.