I don’t know the percentage of the population that feels motion sickness; but as an Audiologist, I can explain what causes it.
Our brain determines our awareness of motion, orientation in space, and equilibrium by combining information from different parts of our body. The fluid in the labyrinth of the inner ear shifts when we move; our bones and muscles send information to the brain as they move or respond to changes by shifting our weight; and our eyes give us visual confirmation of what the other senses experience.
So when we ride in a car, the pull is is felt in our bodies and inner ears; but if our eyes are not confirming the motion, because focusing on a book prevents them from seeing the movement, the brain gets mixed signals and that triggers the nausea of motion sickness. For many people, vision is the dominant of those senses, so they are more likely to have this experience.
I don’t know if there is research to support it, but I have observed over the years that with our bodies, if you don’t use it, you lose it. When we are kids, we run around doing tumblesaults and cartwheels, swinging and flipping on monkey bars, riding bicycles and skateboards, going on amusement rides, swimming in the pool, etc. We put our still-developing vestibular senses through a lot, and they work efficiently (provided they are healthy).
As we get older, many of us become less active in our jobs and recreation activities. Our vestibular system encounters fewer challenges, and so it loses its ability to respond as effectively when we do challenge it. Those who remain more active for more years will be less likely to feel motion sickness. Of course, there are variances across the population, since our genetic strenghths and weaknesses and other medical conditions will make a diiference.
Personally, as I kid I would swing for hours, I loved sitting backwards on the train or in the station wagon… now if I try those things, I feel my stomach tighten-up. Not wanting to feel nauseous, I avoid those situations, so my system gets challenged less and less… which accelerates the downward spiral of the aging process.
Again, I can’t quote any supporting research. This is what I have observed from my own aging process and from working with the elderly with hearing and balance disorders.