Grammatical cues are largely, but not completely, redundant with word meanings in natural language

The combinatorial power of language has historically been argued to be enabled by syntax: rules that allow words to combine hierarchically to convey complex meanings. But how important are these rules in practice? We performed a broad-coverage cross-linguistic investigation of the importance of grammatical cues for interpretation. First, English and Russian speakers (n=484) were presented with subjects, verbs, and objects (in random order and with morphological markings removed) extracted from naturally occurring sentences, and were asked to identify which noun is the agent of the action. Accuracy was high in both languages (~89% in English, ~87% in Russian), suggesting that word meanings strongly constrain who is doing what to whom. Next, we trained a neural network machine classifier on a similar task: predicting which nominal in a subject-verb-object triad is the subject. Across 30 languages from eight language families, performance was consistently high: a median accuracy of 87%, comparable to the accuracy observed in the human experiments. These results have ramifications for any theory of why languages look the way that they do, and seemingly pose a challenge for efficiency-based theories: why have grammatical cues for argument role if they only have utility in 10-15% of sentences? We suggest that although grammatical cues are not usually necessary, they are useful in the rare cases when the intended meaning cannot be inferred from the words alone, including descriptions of human interactions, where roles are often reversible (e.g., Ray helped Lu/Lu helped Ray), and expressing non-canonical meanings (e.g., the man bit the dog). Importantly, for such cues to be useful, they have to be reliable, which means being ubiquitously used, including when they are not needed.