nod of acknowledgement

The all fabled "nod of acknowledgement is an interesting phenomenon. It is, put simply a slight head nod exchanged between males. While its use is documented among females, it is a rare and condition specific phenomenon. While exchanged between friends who, for whatever reason may not want to acknowledge each other verbally, in some places of the world, it is a common greeting among strangers. For example, in the southern United States, it is very common to offer a head nod to another male as you pass each other or get caught glancing at each other. However, as you travel further north, the head nod disappears among unfamiliars. While taking a stroll in NYC, you'd be hard pressed to get any unfamiliar male to return a nod of acknowledgement while simply walking down the street.