Here's why you get carsick — even when other people don't

Life isn't fair. One of the clearest instances of this is the mysterious illness scientists call kinetosis — which most people know as motion sickness.

Some people seemingly start to feel sick the instant they sit down in the passenger seat. If you're one of them, a winding road, a fast-accelerating driver, or being in the back seat is enough to make you utterly nauseated.

Then there are the lucky ones immune to such ills. They sit in the back seat and read for hours as the road winds back and forth, then spring up when the ride is over, oblivious to your misery.

What accounts for the difference? The frustrating truth is that we don't really know. Scientists have found that women and people who get migraines are way more likely to get motion sick, but we don't fully understand why — and we don't even know what causes motion sickness in the first place.

The true cause of motion sickness is still a mystery

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Conventional wisdom holds that motion sickness — whether it occurs in a car, a boat, or a plane — is the result of a mismatch in the signals your brain receives from your eyes and your body systems that indicate movement through space.

When you're in a car looking down at a book, the thinking goes, your eyes are telling your brain that you're not moving. But your vestibular system (a series of structures in your inner ear) thinks you're moving forward and turning left and right as the car moves, explains Timothy Hain, a Northwestern neurologist who studies dizziness and motion sickness.

Some scientists have suggested the brain responds by triggering nausea for an evolutionary reason: Historically, this sort of signal mismatch might have been most often caused by a toxin-induced hallucination, so vomiting up the toxin would have been the best solution.

But there's no real evidence for this argument — and some scientists say the mismatch idea itself is wrong. Thomas Stoffregen, a University of Minnesota kinesiologist, has advanced the main competing theory: that motion sickness is a symptom of the body's inability to maintain proper posture and control in a moving environment.

"Ships are an obvious example," he says. "When the floor is rolling and pitching under you, there will be some body movement that is not under your control." That uncontrolled swaying, he argues, leads to motion sickness.

For evidence, he points to studies he's conducted on ships at sea, with virtual reality displays, flight simulators, and video games, and in a custom-built, nightmarish, nausea-inducing moving room in his lab. "In each and every case, we have found that objective patterns in body sway data differ between people who get sick and people who do not get sick, and that those differences exist before anyone feels sick," he says.

Women and migraine sufferers get motion sick more easily

Yet another motion sick stock photo model. (Shutterstock)

It's uncertain which of the reasons behind motion sickness is correct. But we do know a few factors that can make people more or less prone to suffer from it:

1) Gender: Many different studies have found that women experience motion sickness far more often than men. Part of it may have to do with genetics.

A study published earlier this year using 23andMe data, for instance, identified 35 different genetic variants that were more common in people who frequently get motion sick. Many of them, however, appeared to have larger effects in women — with one particular variant three times more likely to be associated with motion sickness in women than in men. This likely has something to do with the way these genes function in women, but it's unclear how.

In some circumstances, the different body sizes of men and women could also play a role. Stoffregen argues that if swaying and instability cause motion sickness, the fact that women are generally shorter than men — and have a lower center of gravity — could cause them to sway more in unstable conditions. In lab experiments, he's found that women, on the whole, do indeed sway more, and those who sway the most are more likely to experience motion sickness.

2) Migraines: It's well-established — from both questionnaires and lab studies — that people who get migraine headaches are way more likely to suffer from motion sickness. Hain estimates that migraine sufferers are about five times more likely to get motion sick in a given situation.

This, too, is a mystery, but it might also have something to do with genetics. The same 23andMe study found a few genetic variants shared by both migraine and motion sickness sufferers. A number of scientists have proposed that both illnesses might actually share a common underlying mechanism, but at this point, it's unclear — especially since the root causes of migraines aren't understood either.

3) Other genetic factors: It appears that a number of other genetic variants are linked to motion sickness, too.

A 2006 study comparing genetic and identical twins estimated that as much as 57 percent of the variation in people's tendency to get motion sick was from genetics. The 23andMe study, meanwhile, identified several related to the body's balance and visual systems, as well.

How you can minimize motion sickness

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None of these factors, alas, are things you can change. But experts say there are a few things you can do to reduce your odds of getting motion sick.

1) Be the driver: In a car, drivers tend to get motion sick far less often than passengers, perhaps because they're able to anticipate movement. If you're not driving, Hain recommends acting as much like the driver as possible — sitting in front and looking in the direction of travel.

2) Minimize movement: Stoffregen recommends consciously using the head rest, in order to anchor your head in place and minimize the amount of left-and-right swaying. Whatever vehicle you're in, try to sit in a stable position. For example, the middle of a plane generally experiences the least severe turbulence.

3) Don't drink alcohol, read, or sit backward: Doing any of these makes it harder to sync up the stimuli from your eyes and vestibular system — and makes it more difficult to remain stable.

4) Take medication — but it'll make you drowsy: Dimenhydrinate and meclizine (which are used in various forms of Dramamine) can both be effective in reducing motion sickness, but they both mainly work by making you drowsy, dulling your overall sensitivity to movement. Take with caution.

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