The prevailing theory of what causes the dizziness, headaches, and nausea of motion sickness is that riding in vehicles, or on camels, causes confusion between some of the senses. To keep our balance while navigating the world, humans use their eyes, ears, feet, maybe hands if they’re babies who can crawl. The inner ear is the seat of the vestibular system, which deals with movement and balance. And if someone’s eyes tell him one thing—“I am sitting still in a car,” for instance—and his ears tell him another—“I’m careening down the Autobahn at 100 miles an hour”—that mismatch can cause a problem.

“Another place might be, you’re sitting in a movie theater, and [onscreen] you see a plane going over a cliff. Your eyes are telling you you’re in free flight, but the seat of your pants is saying you’re not going anywhere,” says Timothy Hain, an otoneurologist and professor emeritus at Northwestern University, describing another mismatch scenario in which people get motion sick—while looking at screens.

Tom Stoffregen, a professor of kinesiology at the University of Minnesota, has a different explanation. He calls it postural stability theory.

“Never mind the contents of the mind,” he says. “We need to look at the actual physical motions.”

Understanding postural stability theory starts with knowing that the human body is never totally motionless. Even to stand still, muscles have to be active. “If you relax all your muscles, you collapse on the floor,” Stoffregen says. This activity results in a little bit of movement, called body sway. It’s not enough to be noticeable—an inch or two in each direction, a little at a time. Sometimes in yoga classes, teachers will tell you to lean forward onto your toes, then back onto your heels, then return to the center, to get your balance. That’s what people do all the time, just on a smaller scale.

Put those people on a ship, though, and tried-and-true methods of balance often fail. “Let’s say you're standing and your body goes forward,” Stoffregen says. “You need to press with your toes, but what if you do that just as the ship is rolling out from under you? That means that [after] the movement you spent your whole life using to successfully stabilize yourself, your body will move in ways you don’t want it to move. You’ll no longer have the relationship between postural movement and postural outcome.” That instability, by this logic, is what leads to feeling ill.

Stoffregen says these two ways of conceptualizing motion sickness are “just not on the same planet,” but they seem somewhat compatible to me. Both involve a dissonance between expectations and reality, between what the body is used to and what it’s experiencing.

“I’m a little dubious,” Hain says, but “it doesn’t sound tremendously different.”

The genetics data from 23andMe provides some support for both. The researchers found associations between motion sickness and genes involved in eye and ear development, as well as balance.