Nod (gesture)
A nod of the head is a gesture used in many cultures. It is most commonly (but not universally) used to indicate agreement, acceptance, or acknowledgement.
Different cultures assign different meanings to the gesture. Nodding to indicate "yes" is widespread, and appears in a large number of diverse cultural and linguistic groups. Areas in which nodding generally takes this meaning include China, Western Europe, and North America. This usage is not, however, universal. In some countries of Eastern Europe (such as Bulgaria), and in Sri Lanka, nodding can actually has the opposite meaning: "no". It is possible that with the spread of Western culture, the Western use of the gesture has surplanted local usage in certain places.
There are varying theories as to why nodding is so frequently used to mean "yes". One simple theory is that it is a form of bowing, indicating that one is prepared to accept what another person is saying or requesting. It has also been stated that babies, when hungry, search for their mother's milk by moving their heads vertically, but decline milk by turning their head from side to side. This has led some to speculate that nodding for "yes" is at least partially innate, although an explanation for some cultures' divergence would then be required.
An early survey of nodding and other gestures was The Expression of the Emotions in Man and Animals, written by Charles Darwin in 1872. Darwin wrote to missionaries in many parts of the world asking for information on local gestures, and concluded that nodding for "yes" was common to many different groups.