Traveling makes us feel sick because modern transport tricks the brain into thinking we have been poisoned, a neuroscientist has said.
Being in a car, train, boat or plane causes conflicting signals in the brain which trigger a reaction similar to that which occurs when someone is poisoned.
Dr Dean Burnett, of Cardiff University, said the feeling of nausea is caused because the brain thinks the body needs to remove a toxin through vomiting.
But in fact, the 'poisoning' effect is caused by the mixed messages from the muscles – which tell the brain the body is motionless – and the ears, which sense movement.
Speaking on the US radio show Fresh Air, Dr Burnett said that the body had not yet evolved to cope with the sensation of being in vehicles, where the body is being moved without performing movements itself.
He said: 'When we're in a vehicle like a car or a train or a ship especially, you're not actually physically moving... Your muscles are saying 'we are stationary'.
'If you are sitting in a ship, you're looking at a static environment, so there's no information for the eyes to say 'we are moving'.
'But the fluids in your ears, they obey the laws of physics. And they are sort of rocking around and sloshing because you are actually moving.
'So what's happening there is the brain's getting mixed messages. It's getting signals from the muscles and the eyes saying "we are still" and signals from the balance sensors saying 'we're in motion'.
Both of these cannot be correct. There's a sensory mismatch there.
'And in evolutionary terms, the only thing that can cause a sensory mismatch like that is a neurotoxin or poison. So the brain thinks, essentially, it's been being poisoned.
'When it's been poisoned, the first thing it does is get rid of the poison, aka throwing up.'
He explained that reading in a car made the sensation of travel sickness worse, because the eyes were focused on a small, static space and gave the brain no information to explain that the body was moving.
The feeling of sickness could be relieved by looking out of a car window because this showed the brain movement was taking place.
'You can see the passage and movement itself, so that balances the system,' he said.
'The brain's going: 'Oh, look, things moving - I must be moving' - and then sort of calms down the sickness response.'
Dr Burnett, who was discussing his new book 'Idiot Brain: what your head is really up to', said brain systems became more refined and efficient as people aged but that children were more susceptible to travel sickness because their brains were still developing.
He said there was no clear reason why some people suffered from travel sickness more than others, calling it a 'quirk of development'.
But he said there were several other aspects of modern life with which the brain had not yet evolved to cope.
For example, jet-lag was the brain's response to being disorientated by being moved between time zones with different levels of daylight, he said.