Hearing voices can be a sign of a malady, but for many it’s just part of thought. Illustration by Leo Espinosa

“Talking to your yogurt again,” my wife, Pam, said. “And what does the yogurt say?”

She had caught me silently talking to myself as we ate breakfast. A conversation was playing in my mind, with a research colleague who questioned whether we had sufficient data to go ahead and publish. Did the experiments in the second graph need to be repeated? The results were already solid, I answered. But then, on reflection, I agreed that repetition could make the statistics more compelling.

I often have discussions with myself—tilting my head, raising my eyebrows, pursing my lips—and not only about my work. I converse with friends and family members, tell myself jokes, replay dialogue from the past. I’ve never considered why I talk to myself, and I’ve never mentioned it to anyone, except Pam. She very rarely has inner conversations; the one instance is when she reminds herself to do something, like change her e-mail password. She deliberately translates the thought into an external command, saying out loud, “Remember, change your password today.”

Verbal rehearsal of material—the shopping list you recite as you walk the aisles of a supermarket—is part of our working memory system. But for some of us talking to ourselves goes much further: it’s an essential part of the way we think. Others experience auditory hallucinations, verbal promptings from voices that are not theirs but those of loved ones, long-departed mentors, unidentified influencers, their conscience, or even God.

Charles Fernyhough, a British professor of psychology at Durham University, in England, studies such “inner speech.” At the start of “The Voices Within” (Basic), he also identifies himself as a voluble self-speaker, relating an incident where, in a crowded train on the London Underground, he suddenly became self-conscious at having just laughed out loud at a nonsensical sentence that was playing in his mind. He goes through life hearing a wide variety of voices: “My ‘voices’ often have accent and pitch; they are private and only audible to me, and yet they frequently sound like real people.”

Fernyhough has based his research on the hunch that talking to ourselves and hearing voices—phenomena that he sees as related—are not mere quirks, and that they have a deeper function. His book offers a chatty, somewhat inconclusive tour of the subject, making a case for the role of inner speech in memory, sports performance, religious revelation, psychotherapy, and literary fiction. He even coins a term, “dialogic thinking,” to describe his belief that thought itself may be considered “a voice, or voices, in the head.”

Discussing experimental work on voice-hearing, Fernyhough describes a protocol devised by Russell Hurlburt, a psychologist at the University of Nevada, Las Vegas. A subject wears an earpiece and a beeper sounds at random intervals. As soon as the person hears the beep, she jots notes about what was in her mind at that moment. People in a variety of studies have reported a range of perceptions: many have experienced “inner speech,” though Fernyhough doesn’t specify what proportion. For some, it was a full back-and-forth conversation, for others a more condensed script of short phrases or keywords. The results of another study suggest that, on average, about twenty to twenty-five per cent of the waking day is spent in self-talk. But some people never experienced inner speech at all.

In his work at Durham, Fernyhough participated in an experiment in which he had an inner conversation with an old teacher of his while his brain was imaged by fMRI scanning. Naturally, the scan showed activity in parts of the left hemisphere associated with language. Among the other brain regions that were activated, however, were some associated with our interactions with other people. Fernyhough concludes that “dialogic inner speech must therefore involve some capacity to represent the thoughts, feelings, and attitudes of the people with whom we share our world.” This raises the fascinating possibility that when we talk to ourselves a kind of split takes place, and we become in some sense multiple: it’s not a monologue but a real dialogue.

Early in Fernyhough’s career, his mentors told him that studying inner speech would be fruitless. Experimental psychology focusses on things that can be studied in laboratory situations and can yield clear, reproducible results. Our perceptions of what goes on in our heads are too subjective to quantify, and experimental psychologists tend to steer clear of the area.

Fernyhough’s protocols go some way toward working around this difficulty, though the results can’t be considered dispositive. Being prompted to enter into an inner dialogue in an fMRI machine is not the same as spontaneously debating with oneself at the kitchen table. And, given that subjects in the beeper protocol could express their experience only in words, it’s not surprising that many of them ascribed a linguistic quality to their thinking. Fernyhough acknowledges this; in a paper published last year in Psychological Bulletin, he wrote that the interview process may both “shape and change the experiences participants report.”

More fundamentally, neither experiment can do more than provide a rough phenomenology of inner speech—a sense of where we experience inner speech neurologically and how it may operate. The experiments don’t tell us what it is. This hard truth harks back to William James, who concluded that such “introspective analysis” was like “trying to turn up the gas quickly enough to see how the darkness looks.”

Nonetheless, Fernyhough has built up an interesting picture of inner speech and its functions. It certainly seems to be important in memory, and not merely the mnemonic recitation of lists, to which my wife and many others resort. I sometimes replay childhood conversations with my father, long deceased. I conjure his voice and respond to it, preserving his presence in my life. Inner speech may participate in reasoning about right and wrong by constructing point-counterpoint situations in our minds. Fernyhough writes that his most elaborate inner conversations occur when he is dealing with an ethical dilemma.

Inner speech could also serve as a safety mechanism. Negative emotions may be easier to cope with when channelled into words spoken to ourselves. In the case of people who hear alien voices, Fernyhough links the phenomenon to past trauma; people who live through horrific events often describe themselves “dissociating” during the episodes. “Splitting itself into separate parts is one of the most powerful of the mind’s defense mechanisms,” he writes. Given that his fMRI study suggested that some kind of split occurred during self-speech, the idea of a connection between these two mental processes doesn’t seem implausible. Indeed, a mainstream strategy in cognitive behavioral therapy involves purposefully articulating thoughts to oneself in order to diminish pernicious habits of mind. There is robust scientific evidence demonstrating the value of the method in coping with O.C.D., phobias, and other anxiety disorders.

Cognitive behavioral therapy also harnesses the effectiveness of verbalizing positive thoughts. Many athletes talk to themselves as a way of enhancing performance; Andy Murray yells at himself during tennis matches. The potential benefits of this have some experimental support. In 2008, Greek researchers randomly assigned tennis players to one of two groups. The first was trained in motivational and instructional self-talk (for instance, “Go,” “I can,” “Shoulder, low”). The second group got a tactical lecture on the use of particular shots. The group trained to use self-talk showed improved play and reported increased self-confidence and decreased anxiety, whereas no significant improvements were seen in the other group.