Canadian adults use cues that are part of adult speech (voice quality and mouth movements) to create perceptions of prelinguistic, 3-month-old infants as communicative partners. To what degree do cultural differences modulate social attributions of infants? It is thought that Japanese pay less attention to the visual cues that accompany speech, and so we hypothesised that, compared with Canadians, Japanese adults would be less likely to use mouth movement cues, but equally likely to use voice cues, in forming social perceptions of infants. To test this hypothesis, a video tape of 3-month-old, vocalising infants was presented to three (n = 25) groups of adults in Japan and in Canada. The tape was constructed with dubbing so that all combinations of acoustic (syllabic/vocalic) and visual (mouth moving/not moving) conditions were represented. The conditions of tape presentation differed for the three groups in each nation: Audio/Video, Audio-only, Video-only. Infants were rated on characteristics of social favourability and communicative intent. In the absence of audio cues (Video-only group), adult ratings were influenced by the infant's mouth movements and by the adult's culture. Although both Canadians and Japanese gave higher ratings of social favourability and communicative intent to infants whose mouths moved, the preference for mouth movement was significantly weaker for Japanese. The fact that Japanese paid less attention to features of the mouth of the infant is consistent with past cross-cultural research and with the minor role of facial cues in Japanese Sign Language. Our research demonstrated that adults apply cultural rules of adult conversation when responding to preverbal infants.