Putting the pieces together: The development of theory of mind and (mental) language

A particular milk carton contains a coin. Show someone else the carton and ask them what’s in it. What will they say? Milk! But this is not so obvious for young children. Appreciating other people’s beliefs is hard for them, especially if those beliefs are different from their own. The understanding of others’ beliefs as separate entities develops in most children somewhere between their third and fifth year, at which point they are said to have a “Theory of Mind” (ToM). The relationship between language and ToM is debated. That there is some kind of relationship between ToM and linguistic development is generally accepted, but the precise nature of the relationship is considerably less clear. Is ToM pivotal in the development of language? Or is linguistic development a prerequisite for the development of ToM? Each of the chapters in this dissertation attempts to address these questions by focussing on children’s ToM development in relation not only to their linguistic capacities in a general sense (e.g. general vocabulary and understanding of syntax), but also to their understanding of language that relates to mental states more directly: “mental” language (e.g. understanding of mental state verbs and indirect requests). The developmental relationship between ToM, general language and mental language is investigated in this thesis in three- to six-year-old children, in typically and atypically developing populations, in correlational and longitudinal studies using not only traditional or standardised assessment materials, but novel methods of testing as well. The main finding is that there is a complex developmental relationship between language and ToM, with bi- and unidirectional relations between some domains of language and ToM, but no relationship between ToM and other aspects of language. More specifically, although previous findings in the literature have suggested that children’s understanding of sentential complementation constructions (e.g. John thinks the key is in the cupboard) is fundamental for ToM development, this aspect of language was not found to be related to ToM in this research. Furthermore, longitudinal data demonstrates that ToM and both general and mental vocabulary are bi-directionally related (ToM influences general and mental vocabulary development and general and mental vocabulary influence ToM), but that the child’s general language ability at the sentential level unidirectionally influences ToM development. In addition to these findings, a post-hoc result suggests that understanding of locative prepositions (e.g. next to, behind) may be an important prerequisite for ToM development as well. If replicated, this suggests that children may come to understand that other people’s mental states can differ from their own by first appreciating more tangible differences in perspective as conveyed by locative prepositions (the tree may be next to the house from one speaker’s perspective, but a speaker in a different spatial position may consider it to be behind the house), before more abstract differences in people’s mental states can be comprehended. The development of ToM and language is thus intricately interwoven, but the precise nature of the relationship depends on the particular domain of language that is considered