Scientific American [unpaywalled]: “Most of us have an “inner voice,” and we tend to assume everybody does, but recent evidence suggests that people vary widely in the extent to which they experience inner speech, from an almost constant patter to a virtual absence of self-talk. “Until you start asking the right questions you don’t know there’s even variation,” says Gary Lupyan, a cognitive scientist at the University of Wisconsin–Madison. “People are really surprised because they’d assumed everyone is like them.” A new study, from Lupyan and his colleague Johanne Nedergaard, a cognitive scientist at the University of Copenhagen, shows that not only are these differences real but they also have consequences for our cognition. Participants with weak inner voices did worse at psychological tasks that measure, say, verbal memory than did those with strong inner voices. The researchers have even proposed calling a lack of inner speech “anendophasia” and hope that naming it will help facilitate further research. The study adds to growing evidence that our inner mental worlds can be profoundly different. “It speaks to the surprising diversity of our subjective experiences,” Lupyan says. Psychologists think we use inner speech to assist in various mental functions. “Past research suggests inner speech is key in self-regulation and executive functioning, like task-switching, memory and decision-making,” says Famira Racy, an independent scholar who co-founded the Inner Speech Research Lab at Mount Royal University in Calgary. “Some researchers have even suggested that not having an inner voice may impact these and other areas important for a sense of self, although this is not a certainty.”

Source – Not Everyone Has an Inner Voice: Behavioral Consequences of Anendophasia – It is commonly assumed that inner speech – the experience of thought as occurring in a natural language – is both universal and ubiquitous. Recent evidence, however, suggests that similar to other phenomenal experiences like visual imagery, the experience of inner speech varies between people, ranging from constant to non-existent. We propose a name for a lack of the experience of inner speech – anendophasia – and report four studies examining some of its behavioral consequences. We found that people who report low levels of inner speech have lower performance on a verbal working memory task and have more difficulty performing rhyme judgments based on images. Task switching performance, previously linked to endogenous verbal cueing, was unaffected by differences in inner speech. Studies of anendophasia, together with aphantasia, synesthesia, and differences in autobiographical memory are providing glimpses into what may be a large space of hitherto unexplored differences in people’s phenomenal experience.”