Maybe it's the sight of blood, or standing too much, or pain, or scary thoughts - in any of these cases, you might find yourself alert one second and unconscious the next.

About 25% of people will faint during their lifetime. The technical term for fainting, when it happens because of an environmental trigger, is "vasovagal syncope." The process involves a drop in blood pressure.

A new study in the journal Neurology suggests that there is a genetic component to fainting.

Previously, there had been mixed evidence about the role that genetics plays in the tendency to faint. A recent review of studies suggested that any familial correlation was due to chance.

But this study looks at twins, who share more genetic material than any other two people. Researchers looked at 51 same-sex pairs of twins, where at least one had fainted. These included a mix of monozygous (derived from one egg) and dizygous (derived from two eggs) twins. Monozygous twins are more genetically similar and are sometimes said to be "identical," while dizygous twins are called "fraternal."

Researchers interviewed the twins on the phone with a standardized questionnaire. The results suggested that in the twin pairs, both identical twins were more likely to have fainted than both fraternal twins.

"Twin studies provide very strong evidence of a genetic component and are a classical clinical method to look for evidence of genetic effects. This is backed up by the non-twin family data in our paper," said study co-author Dr. Sam Berkovic, neurologist at the University of Melbourne, Australia, in an e-mail.

The research suggests that there is a strong genetic component in fainting, "making subjects vulnerable to environmental triggers to result in a faint at a particular moment, such as emotional events, stress, sight of blood etc." Berkovic said.

There was no evidence that participants had heart problems or autonomic failure (a nervous system disorder) causing fainting, and no twin pair reported a family history of sudden cardiac death.

It appears that there is a spectrum of fainting causes, the study said. On one side, there's genetic factors, particularly when recurrent and associated with typical triggers. On the other side, there are environmental factors, especially "when infrequent and not associated with typical triggers."

This means that people who are closely related may have a similar tendency to faint, Berkovic said. But keep in mind that the study has several weaknesses. This was a small study, and the researchers did not perform diagnostic tests to exclude other causes of fainting, such as heart problems and autonomic failure.

The research could help scientists identify the genes underlying a tendency to faint, Berkovic said.

To further explore this topic, there should be genetic studies on large families examining the tendency to faint, he said.