Originally Posted by modelman
I reckon you will find it like some people car sickness-passenger=sick
driver=too busy to even think about being sick.
That may play a part but, from what I've experienced (and read), there's more to it than that. In terms of cars, research shows a driver whilst driving naturally moves his/her head in a way that minimises the risk. From my experience on buses, I find that having a good view ahead helps significantly (which is why I'd never sit on the right hand side of a bus[1]).
[1] In the UK anyway.
{edit} Just had a thought (though may be wrong):
Where's the balance point (not sure of the technical term) of a C172? If you're in the back, and the pilot pulls back on the yoke, would the passenger go up or down?
If the passenger moves downwards, then you'll be in a situation where the balance organs say you're descending but your eyes say you're climbing which can, for some people, induce travel sickness.