What would prevent a cop from ticketing you for reckless driving to increase the funding of his police department despite the fact the way you drove was textbook perfect ? <| sep |> While neither one is universal , nodding is an incredibly common indication of yes and shaking is an incredibly common indication of no and we really don ' t have a good reason for either . There are plenty of theories for each , though . One theory for how nodding came to mean yes says that it is an evolved form of a bow , which is traditionally used to indicate yes , I accept whatever you are sayingdoing . One theory for how head shaking came to mean no says that babies will often move their heads vertically when searching for milk and horizontally when rejecting it . Neither of these is 100 %, and it ' s unlikely that we ' ll ever know for sure . Interestingly enough , in some cultures the meanings are reversed shaking indicates yes and nodding indicates no