Daily activities of forty-eight 8- to 15-month-olds and their interlocutors were observed to test for the presence and frequency of triadic joint actions and deictic gestures across three different cultures: Yucatec-Mayans (Mexico), Dutch (Netherlands), and Shanghai-Chinese (China). The amount of joint action and deictic gestures to which infants were exposed differed systematically across settings, allowing testing for the role of social-interactional input in the ontogeny of prelinguistic gestures. Infants gestured more and at an earlier age depending on the amount of joint action and gestures infants were exposed to, revealing early prelinguistic sociocultural differences. The study shows that the emergence of basic prelinguistic gestures is socially mediated, suggesting that others' actions structure the ontogeny of human communication from early on. (Contains 5 figures and 1 table.)