Search Efficiency Drives Reference Production Across Modalities, But Colour Is Special.

When speakers refer to objects in the world, they frequently overinform. Contrary to classical theories in linguistics, we hypothesise that overinformativeness is an efficient means of facilitating listener comprehension: speakers use redundancy to provide their listeners with search-efficient perceptual information. In Experiment One (N = 72), we operationalise search efficiency as the ease or difficulty of perceptual discrimination. We borrow methods from psychophysics to manipulate discriminability across attributes (material, colour) and across sensory modalities (audition, vision). We show that across both attributes and modalities, speakers overinform more often when the redundant information helps the listener perceptually discriminate the referent, thus aiding listener-search. Contrary to our expectations, we also find that speakers disproportionately overinform with colour information (relative to material). In Experiment Two (N = 97), we investigate the disproportionate use of colour directly, addressing explanations that appeal to colour's perceptual salience by 1) dampening colour's perceptual distinctiveness, and 2) controlling linguistic complexity both in terms of language production and retrieval effort. Contrary to widespread explanations, these factors cannot explain the disproportionate use of colour: colour is privileged in reference.