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

Languages have evolved in part due to crossmodal associations. 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 & 2, we created standard Bouba-like and Kiki-like shapes and words for use in subsequent experiments. In Experiments 3 & 4, we created composite shapes/words by combining Bouba-like & Kiki-like parts. In both 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.