This is an interesting topic, one, as a former Navy pilot who cruised on very small frigates in very large seas, I researched. I have only been sick once.
There really are shades of gray in the root cause of motion sickness. The Navy medical establishment acknowledges definitive physical attributes that can contribute to motion sickness, but they also acknowledge definite
mental contributory factors as well: stress, apprehension, prior experiences as a child, anticipation of getting sick, etc.
In flight school there are always hard core pukers, guys and gals that get motion sickness on pre-flight
The Navy will, if the student is willing and has been doing well, try and get the individual on a path back to flight.
They start with the patch, which often works. A good friend of mine who is a flight surgeon told me that if the patch works many times the third or fourth "patch" prescription is basically a placebo...with identical results.
Failing the patch, they move to the physical side, inner ear, blockage, etc. If there are no obvious, non-repairable issues, the fun begins.
Another good friend of mine (spent some time shooting with him and his son this weekend) I met in flight school was a hurler, a dynamic bag filler. The patch failed, he had no physical causes so, as he described it, the torture began.
Spin chairs, lots of time in visual flight sims, g-force trainers (
NASA G-Force Training), etc. and basic counseling. The purpose, as my friend commented, is to introduce you to as many of the worst possible chow-returning scenarios as possible so that flight in an aircraft is welcomed relief.
It worked for him. I have no statistics, but there is often a definitive link between motion sickness and the mind only...that is why driving instead of riding, piloting instead of riding, is often key: the mind concentrates on a task and forgets about the motion.