Communication increases category structure and alignment only when combined with cultural transmission

The semantic categories labeled by words in natural languages are used for communication with others, and learned by observing the productions of others who learned them in the same way. Do these processes of communi-cation and cultural transmission affect the structure of category systems and their alignment across speakers? We examine novel category systems that emerge from communication, cultural transmission, and both processes com-bined. Communication alone leads to category systems that vary widely in their communicative effectiveness, and are no more structured or aligned than those created by individuals. When combined with cultural trans-mission, communication speeds up convergence on a learnable number of structured, aligned categories that are consistently communicatively effec-tive. However, cultural transmission without communication eventually has similar results. Communication appears to be neither necessary nor suffi-cient for creating semantic category systems that are robustly effective for communication. Furthermore, category systems that emerge from cultural transmission are more aligned across speakers than the systems created by individuals, suggesting that cultural transmission allows individuals to coor-dinate their semantic systems more effectively than they can through shared perceptual biases alone.