Fainting may run in families, a study of twins has shown.
Fainting is a brief loss of consciousness caused by a sudden fall in blood pressure which reduces the blood flow to the brain.
A common type of fainting known as vasovagal syncope occurs in response to triggers such as the sight of blood, pain and emotional stress. But why some people are more likely to react in this way than others is unclear.
A new study of twins published in the journal Neurology suggests that a genetic component may be involved.
"The question of whether fainting is caused by genetic factors, environmental factors or a mixture of both has been the subject of debate," said Professor Sam Berkovic from the University of Melbourne, who led the research team.
For the study, the researchers looked at 51 same-sex sets of twins between the ages of nine and 69. At least one of the twins had a history of fainting.
They found that identical twins were more likely to both experience fainting triggered by typical factors such as the sight of blood than non-identical twins, rather than other causes such as dehydration.
As fainting episodes among other family members related to the sets of twins was much lower, this suggests the way fainting is inherited is not down to a single gene but is more complex, the researchers said.
"Our results suggest that while fainting appears to have a strong genetic component, there may be multiple genes and multiple environmental factors that influence the phenomenon," Prof Berkovic said.