Previous research on human head nodding has shown that :
“[...] head nods can function in a range of ways either as initiating actions designed to elicit particular responses from recipients (‘first position’), or as responding actions (‘second position’).”
Professor Kevin Whitehead , BSc, BSc (Hons), MSc, MA, PhD (of the Psychology dept at the University of the Witwatersrand, Johannesburg, South Africa) examines the possibilities of a third type of head nod :
“[...] that embodies features of the first two types, and may be designed to register receipt and acknowledgment of ‘dispreferred’ news.”
The professor notes that there is still room for further research regarding other possible variants and fine-grained sub-variants of as yet undescribed head nod classifications :
“It is important to note that these findings should not be seen as providing an exhaustive typology of all possible different uses of nods in this type of sequential position, but rather as a description of some systematic ways in which they can be used.”
The paper ‘Some uses of head nods in “third position” in talk-in-interaction’ is published in the journal Gesture, 11:2. 2011. iii, 138 pp. (pp. 103–122), and may also be read in full here.