Shape and word parts combine linearly in the Bouba-Kiki effect

Languages have evolved in part due to cross-modal associations between shape and sound. A famous example is the Bouba-Kiki effect, wherein humans associate words like bouba/kiki to round/angular shapes. How does the Bouba-Kiki effect work for natural words and shapes that contain a mixture of features? If the effect is holistic, the effect for a composite stimulus would not be explainable using the parts. If the effect is compositional, it will be. Here we provide evidence for the latter possibility. In Experiments 1 and 2, we standardized bouba-like and kiki-like shapes and words for use in subsequent experiments. In Experiments 3-5, we created composite shapes/words by combining bouba-like & kiki-like parts. In all experiments, the Bouba-Kiki effect strength for composite shapes/words was predicted remarkably well as a linear sum of the contributions of the constituent parts. Our results greatly simplify our understanding of the Bouba-Kiki effect, leaving little room for holism.