Language promotes false-belief understanding: evidence from learners of a new sign language

Abstract

Developmental studies have identified a strong correlation in the timing of language development and false-belief understanding. However, the nature of this relationship remains unresolved. Does language promote false-belief understanding, or does it merely facilitate development that could occur independently, albeit on a delayed timescale? We examined language development and false-belief understanding in deaf learners of an emerging sign language in Nicaragua. The use of mental-state vocabulary and performance on a low-verbal false-belief task were assessed, over 2 years, in adult and adolescent users of Nicaraguan Sign Language. Results show that those adults who acquired a nascent form of the language during childhood produce few mental-state signs and fail to exhibit false-belief understanding. Furthermore, those whose language developed over the period of the study correspondingly developed in false-belief understanding. Thus, language learning, over and above social experience, drives the development of a mature theory of mind.

Fig. 1

Example of one of the live-action videos used to elicit mental-state verbs. Here, a woman’s drink is replaced with a vase of flowers while she is not paying attention. The woman then mistakenly picks up the vase instead of her drink.

Fig. 2

Examples of mental-state verbs in Nicaraguan Sign Language. The signs for (a) know and (b) doesn’t know are shown.

Fig. 3

Examples of the story-card sequences used to assess understanding in the (a) false-belief and (b) true-belief scenarios. In (a), a boy’s toy train is moved from under a bed to a toy box while he is eating in the next room. Participants who selected the false-belief card (looking under the bed) to complete the story indicated a mature understanding of the boy’s false belief. In (b), the boy watches while his brother places the train into the toy box. In this condition, participants who selected the true-belief card (looking in the toy box) to complete the story indicated that they understood what both they and the boy had seen to be true.

Fig. 4

Mental-state language and false-belief understanding by cohort. The graphs on the left show the total number of mental-state verbs produced per signer in the four narratives in (a) 2001 and (c) 2003. The graphs on the right show the percentage of correct trials on the false-belief task in (b) 2001 and (d) 2003; the numbers within the bars indicate the mean number of correct responses (out of four trials in 2001 and out of three trials in 2003). Error bars represent standard errors.