Inner speech refers to the experience of talking to oneself in one's head. While notoriously challenging to investigate, it has also been central to a range of questions concerning mind, brain, and behaviour. Posited as a key component in executive function and self-regulation, inner speech has been claimed to be crucial in higher cognitive operations, self-knowledge and self-awareness. Such arguments have traditionally been supported with examples of atypical development. But variations in inner speech - and in some cases, significant diversity - in fact pose several key challenges to such claims, and raises many more questions for, language, thought and mental health more generally. In this review, we will summarise evidence on the experience and operation of inner speech in child and adult neurotypical populations, autistic people and other neurodivergent groups, and people with diverse experiences of linguistic and sensory development, including deafness. We will demonstrate that the relationship between inner speech and cognitive operations may be more complex than first assumed when explored through the lens of cognitive and neurological diversity, and the implications of that for understanding the developing brain in all populations. We discuss why and how the experience of inner speech in neurodivergent groups has often been assumed rather than investigated, making it an important opportunity for researchers to develop innovative future work that integrates participatory insights with cognitive methodology. Finally, we will outline why variations in inner speech - in neurotypical and neurodivergent populations alike - nevertheless have a range of important implications for mental health vulnerability and unmet need. In this sense, the example of inner speech offers us both a way of looking back at the logic of developmental psychology and neuropsychology, and a clue to its future in a neurodiverse world.

It is plausible to regard inner speech as an activity, whose functions are continuous with those of outer speech. Yet there is also a case for thinking that inner speech has a cognitive role, as a vehicle for conscious thought. This chapter reconciles these apparently conflicting claims by showing how outer speech can itself play a cognitive role. Drawing on dual-process theories of reasoning, it develops a view of conscious (‘Type 2’) thinking as an activity, initially performed in outer speech. By engaging in self-directed speech, we can break down complex problems into subproblems that can be solved by nonconscious (‘Type 1’) processes, thereby vastly extending our reasoning powers. It is as the subsequent internalization of such ‘outer thinking’ that inner speech has a cognitive function. The chapter also extends this account to show how acts of inner speech can function as judgements and decisions.

l linguistic items. 2. Inner Speech and Thought A second question about inner speech is how it relates to thought. It seems that there must be some relationship, but it is an open question what that relationship is. In general, there are three views about the nature of the relationship: (1) inner speech episodes express thoughts; (2) inner speech episodes facilitate thoughts; and (3) inner speech episodes (at least sometimes) are thoughts of a certain kind. The views are not mutually exclusive: one can certainly hold that inner speech is related to thought in multiple ways. 2.1 Inner Speech and Thought Expression Langland-Hassan & Vicente (2018b: 10) observe that the view that inner speech (at least often) expresses thoughts that are distinct from the inner speech episodes themselves coheres with some larger theories about thought and language. If one is attracted to these theories, then they may well also be attracted to the view that inner speech merely expresses thought. First, there is a natural connection between the language of thought hypothesis, most closely associated with Jerry Fodor (1975), and the view that inner speech expresses thought. On the language of thought hypothesis, our thoughts do take place in a language, but not in a natural language. Rather, our thoughts take place in a kind of mental language, often referred to as “Mentalese”. If the language of thought hypothesis is true, then, insofar as inner speech is keyed to a natural language, it seems that inner speech can at most serve to express the thoughts which occur in the mental language. Second, on Willem Levelt’s influential theory about language production, speaking involves conveying a pre-existing “message” (1989: passim). The structure of this message is conceptual but not linguistic. Via several stages of processing, natural language sentences (or sub-sentential items) are formulated which, once articulated, express the conceptually structured message wi