I believe my most important scientific contributions have been in the area of the development of children's knowledge about the mind, often referred to by psychologists as theory-of-mind development or metacognitive development. As so often happens in science, my research on this topic had its origins in work on what seemed at the time to be a different topic, namely, the development of children's private, non-social, speech-for-self.

To study that topic, in the early 1960s, my students and I devised a task in which kindergarteners, second graders, and fifth graders wore a “space helmet” with an opaque visor that could, when lowered, prevent them from seeing a series of object pictures on the table in front of them. The pictures showed objects, the naming of which required rather large, distinctive, easily readable mouth movements (e.g., “pipe,” “apple,” “flag”). On each trial with the visor up, the experimenter pointed in a fixed order to a subset of the pictures and then asked children to point to the same pictures in the same order, either immediately or after a fifteen second delay during which the vision-obscuring visor was lowered.

We found a marked increase with age in children's spontaneous picture naming (which we either heard or lip-read). In particular, unlike the younger ones, most of the oldest children tended to name the objects when they first saw them, repeat the names to themselves while the visor was down, and then try to repeat them again while pointing to the pictures after the visor was lifted.

In this and subsequent studies it became apparent to us (slow learners, we!) that what we were really investigating was the development of tacit knowledge about what remembering was like and how best to accomplish memory goals (e.g., verbal rehearsal); we gave the name “metamemory” to this domain of naïve knowledge about memory. It was then but a short step to conceptualize (with the help of independent work by the late Ann Brown) the area more broadly as any kind of cognition about any kind of cognition – that is, as “metacognition.” Thus it was that we began to trace the development of naïve knowledge about other forms of cognition.