If catching sight of blood or standing all day makes you woozy enough to black out, your genes are partly to blame. Fainting, like dimples and dyslexia, can run in the family, a new study shows.
Fainting is fairly common -- nearly one in four people experience it at least once during his or her lifetime. But researchers have long debated whether this behavior is written into our DNA or is more influenced by the environment around us.
Enter the identical twins. Studies of such siblings, whose genetic material is almost exactly the same, are one of the surest ways to figure out how much genes influence a particular trait. (This National Geographic twins story delves into the topic.)
For this particular study, reported Tuesday in the journal Neurology, researchers in Australia and Germany put out a call for volunteers to the Australian Twin Registry, an organization that helps connect twins with medical and scientific researchers. Researchers recruited 36 pairs of identical twins and 21 pairs of same-sex fraternal twins (who are no more similar genetically than a regular brother or sister). In every pair, at least one twin had a confirmed fainting episode.
Researchers phoned up the twins and asked each about their fainting history, whether they had particular triggers or forewarning symptoms, and if any other family members also fainted.
In all, 57% of the study subjects said they reacted to typical fainting triggers, such as the sight of blood, injury and pain, medical procedures, prolonged standing or scary thoughts. Others said that additional factors brought on their fainting, such as illness or dehydration.
Pairs of identical twins were much more likely to both experience fainting than were pairs of fraternal twins, the researchers reported. This was especially true for fainting associated with common triggers, and for frequent fainters (those who’d experienced three or more fainting episodes).
Since identical twins have essentially the same genes, and fraternal twins share only 50% of their genes on average, this make a strong case that fainting is partly genetic.
The researchers don’t think the tendency to faint is controlled by a single “fainting” gene, however. If that had been the case, they would have been able to trace the inheritance of that single gene as it was passed down through each twin pair’s family tree. Instead, the researchers suspect it’s a complex trait, like height or skin color, that results from multiple genes and environmental factors.
The study authors say the reasons for fainting probably span a spectrum from mostly genetic to mostly environmental, depending on the individual. Their results suggest that genes might play a larger role in people with more frequent fainting spells or who succumb to typical triggers such as blood or pain. Environmental factors may have a larger influence among those who faint infrequently and for less common reasons such as dehydration.
So if you’re prone to fainting, best not to put your identical twin in charge of the smelling salts -- chances are, he’ll be needing a whiff as well.