I’d like to hazard a guess that when you talk to yourself out loud, things are just a little bit more difficult or challenging or stressful than usual. I think we particularly start to say things out loud when the going gets tough. That certainly fits with how private speech works with children, children will talk to themselves more when things are more difficult.

The very fact that adults do talk to themselves does suggest we need to rethink that bit of Vygotsky's theory. Although the inner speech in our heads comes from that social language initially, and then this out loud private speech, when it goes underground, it can come back out again. It doesn't go underground permanently. It's not a one-way street. I would say most people talk to themselves, but there's still a sort of social embarrassment about doing it.

It’s quite nice to speculate about why we do this from an evolutionary point of view. When we acquired language and when we started to use language in out-loud private speech, we'd have learned pretty quickly that it’s not a good idea to talk to yourself out loud when you're in a difficult dangerous situation. We wouldn’t have lasted very long with that saber-toothed tiger if we were muttering away to ourselves in the bushes. And then there's a sort of social and cultural pressure as well. If you're going around saying what you think, your competitors, your rivals, the other people around you will know what you’re thinking and then it’s hard to fulfill your plans. So there are some good reasons for doing it silently.

Beck: Of course, most of the situations we're doing it in now are not that extreme. It's funny, I always find I talk to myself out loud most at the grocery store. Just something about the grocery store stresses me out, all the people looking at you while you're trying to buy your food.

Fernyhough: Although this is solitary speech, it's speech for the self, it seems to be stimulated by the presence of other people. Children do it more when there are other kids around. And I think that might apply to adults as well—if you're in a context where everybody else is muttering to themselves, [you might, too]. I do it in the supermarket because I'm trying to remember the last things on the list.

Beck: Or you can't find something, it's supposed to be over here but it's not.

Fernyhough: There's a neat study that shows that kind of self-talk actually helps you do exactly that—pick items from a supermarket array. That’s one of the benefits that's been proven for self-talk.

Beck: You mention that part of Vygotsky's theory is that as we're learning social speech, we're also learning internal speech. Walk me through: How does the development of spoken language correspond with the development of inner speech?

Fernyhough: So Vygotsky thought that two things come together in early childhood. You have some basic intelligence, which any one-year-old baby is showing. They're able to do all sorts of things, initiate actions, work stuff out, remember stuff. But that's intelligence before language—it’s prelinguistic intelligence. And then you’ve got this thing that comes along which is language. It's quite phenomenal how quickly most kids acquire language. The idea is not that you need language for thinking but that when language comes along, it sure is useful. It changes the way you think, it allows you to operate in different ways because you can use the words as tools. Somewhere around age 2, language comes together with intelligence and bang! Something really special is created. And the thing that is created might well be unique in the universe.