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When Scarlett O'Hara swoons into the arms of Rhett Butler, it is likely to be because the part of her brain that processes emotions is a little on the small side.

New research by Brighton and Sussex Medical School and Imperial College London has found that people who faint, which is about a third of the population, have a different brain anatomy to non-fainters.

Neuroscientists used MRI brain scanning to examine brain anatomy in people with histories of fainting, and those who had never fainted. They found that the fainters had smaller regions of the brainstem (medulla and midbrain), which control blood flow to the brain.

Fainting is caused by a dramatic reduction in blood flow to the brain. But this is sometimes triggered by purely emotional factors, such as the sight of blood. Consistent with this, the researchers also found that those more prone to blacking out have smaller volume of a brain region known as the caudate nuclei, which is thought to be involved with emotional processing and anxiety disorders.

Researcher Dr Felix Beacher, who is based at the Trafford Centre on the Sussex campus, says: "Fainting, which affects up to 35% of people at least once in their lives, involves an interaction between the mind (as in fainting at the sight of blood), the brain, which regulates blood flow, and the heart, which pumps blood to the brain.

"Previous research has shown a connection between blood flow and the brain as a cause of fainting, but this is the first study to demonstrate that this is related to differences in brain anatomy."

Fainting is thought to be a survival mechanism that evolved in mammals to counter the effects of blood loss. Animals are likely to lose less blood if lying down still. However, humans are particularly prone to fainting because, unlike most other mammals, they stand upright and therefore it is harder for the body to pump blood to the brain.

Felix says: "Some people have clinical conditions which are more likely to make them faint. But we were not looking at that group of people - we were looking at healthy people with non-clinical fainting. It is most likely that these differences in brain anatomy simply reflect normal variations, which make some people more vulnerable to fainting.

"In the right circumstances anyone can be made to faint - just stand someone upright for long enough! But some people will faint more quickly than others."

The study, 'Vulnerability to simple faints is predicted by regional differences in brain anatomy', is published in NeuroImage