A new study of twins show that a common kind of fainting might run in the families. The Genetic Link Researchers in Australia interviewed 51 same-sex sets of twins for the study. Some of the twins were identical. The others were fraternal twins. At least one twin in each pair reported passing out at some point in their lives.

Researchers found that both members of a set of identical twins were more than twice as likely to be prone to fainting spells compared to fraternal twins. Their fainting was also more likely to be triggered by something external, like a stressful event or the sight of blood, rather than by a biological problem like dehydration.

Fainting even though usually not dangerous is caused by sudden, brief loss of consciousness after blood pressure drops to the brain. Sometimes, that loss of blood pressure happens for internal reasons -- dehydration or heart problems, for example. But puzzlingly, people sometimes black out in response to some kind of outside-the-body trigger, like the sight of blood or after some kind of emotional upset. This is called vasovagal syncope. People wake up a few seconds after the fainting spell but might fall in the process which can led to injuries, serious or non-serious.

Various neurological studies have shown that fainting can be inherited in some cases and if the genes are identified that lead to it they may be able to prevent fainting to some extent. "A small proportion of people with fainting actually have a disabling problem. They have a lot of fainting attacks, and it makes life a little challenging for them. And if we can develop better treatments for that, that would be great," says researcher Samuel F. Berkovic, MD, a neurologist and professor in the department of medicine at the University of Melbourne, in Australia.