From Paul Stancioff:

Re: Scientists explore whether some apes shake heads for “no” (May 5): Implicit in this article is the assumption that head shaking by humans universally denotes negation, further implying that it is somehow instinctive or evolutionary. This assumption is certainly wrong, as a simple counterexample will show. In Bulgaria for example a head shake always means yes, while a nod is a definite “NO”.