Human beings often make frequent hand movements whilst they are talking. There has been considerable psychological debate about their function, but it is now clear that they often convey core parts of the underlying message. Since we have little conscious awareness of these movements, they can be particularly revealing.
We control what we say, but find it difficult, or impossible, to control the content and form of these movements. Their form and ‘meaning’ may not match the accompanying speech and these gesture-speech mismatches can indicate various underlying psychological states, including deception. I will argue for the essential unity of speech and gesture in the transmission of thought, and suggest that we have underestimated the considerable communicative significance of these movements.