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Possession of a "theory of mind" (ToM)--as demonstrated by an understanding of the false beliefs of others--is fundamental in children's cognitive development. A key question for debate concerns the effect of language input on ToM. In this respect, comparisons of deaf native-signing children who are raised by deaf signing parents with deaf late-signing children who are raised by hearing parents provide a critical test. This article reports on two studies (N = 100 and N = 39) using "thought picture" measures of ToM that minimize verbal task-performance requirements. These studies demonstrated that even when factors such as syntax ability, mental age in spatial ability, and executive functioning were considered, deaf late signers still showed deficits in ToM understanding relative to deaf native signers or hearing controls. Even though the native signers were significantly younger than a sample of late signers matched for spatial mental age and scores on a test of receptive sign language ability, native signers outperformed late signers on pictorial ToM tasks. The results are discussed in terms of access to conversation and extralinguistic influences on development such as the presence of sibling relationships, and suggest that the expression of a ToM is the end result of social understanding mediated by early conversational experience. [ABSTRACT FROM AUTHOR]