Bobbing Heads as communication

[Martin Taylor 930325 12:00]
(Gary Cziko 930323.1700)

You don't need an elevator and don't need to travel alone. Just watch
people's head bob back and forth in a car (or bus or train or whatever) in
"reponse" to changes in acceleration. As driver when I slow down to make a
stop I see my passengers' heads move forward then back again to where it
should be. As I accelerate from the stop, the heads move backwards before
coming to rest again a la verticale. No problem with explaining this using

But what is particularly intriguing is that the driver's head does not seem
to move nearly as much.

What (to me) is even more interesting is that this is a well-known
conventionalized communicative signal. In the days before automatic
transmissions, one sign of a good driver was the ability to make smooth
gearshifts. If a passenger wished to comment on the lack of skill of
the driver, he would allow his (usually male) head to bob quite visible.
In PCT terms, he would either increase his transport lag or reduce his
gain (I suspect the former, since the head would return quickly to its
normal position.) The driver might well feel insulted by this behaviour
(I don't know whether to use behaviour or action in this instance).