Proceedings of the Annual Meeting of the Cognitive Science Society

of human language. We focus on the debate between gesture-first and vocalization-first theories. While some evidence supports the idea that gestures played a primary role in early communication, others argue that vocalizations are equally expressive. We think that methodological differences and biases in the choice of concepts may contribute to the challenge of comparing these modalities directly. For example, to what extent does selecting a certain concept from a semantic category matter to reproduce an effect? This and similar questions are explored in a data-driven way. First, we provide ratings on imagined expressibility of 207 concepts from an online experiment showing that people tend to rate gesture modality as better in expressing meaning compared to vocal modality. Second, we use the Bayesian posterior predictive distribution of these ratings to simulate new experiments where we vary the number of participants, number of concepts, and semantic categories to investigate how robust is the difference between gesture and vocal modality. Our results show that gesture modality is reliably different (i.e., affords higher expressibility) than vocal modality. However, the difference between the two is limited in terms of effect size (medium sizes by common standards) so one may question whether this difference is meaningful for bigger claims about early language evolution. This study further provides valuable information for further research on how to select stimuli and how to set up one's design in a balanced way.