Fainting may be in our genes, which may explain why keeling over at the sight of blood tends to run in families, according to researchers in Australia.
The researchers located a specific region on chromosome 15 that is thought to be a prime suspect for "vasovagal syncope," a drop in blood pressure followed by loss of consciousness.
The study "strengthens the evidence that fainting may be commonly genetic," said neurologist Samuel Berkovic of the University of Melbourne in Victoria, Australia, author of the report published this week in the journal Neurology.
Berkovic's team interviewed 44 families with a history of fainting, six of them with a significant number of relatives who faint. One family included 30 people in three generations.
What triggered the fainting? Pretty predictable stuff, such as the sight of blood, injury, a medical procedure, pain, frightening thoughts and prolonged standing.
Scientists have been debating whether fainting is genetic, environmental or both. Some suspect fainting may have an evolutionary benefit; falling over and lying still in response to blood loss increase survival chances.
Last year, Berkovic published findings of a study that showed identical twins were twice as likely to share the fainting trait than were fraternal twins. Fainting among nontwin relatives was low, suggesting that the way fainting is inherited is usually not by a single gene.
Other research suggests there may be a genetic link that makes people susceptible to blood phobias, but that substantial environmental factors come into play.
A person may have a predisposition to react strongly to trauma, such as the sight of blood, with a heart-pounding "fight or flight" reaction, then with a counterreaction that slows down the pulse and blood so much that it causes loss of consciousness.
And there's nothing like passing out, or watching Mom or Dad pass out, to teach the brain to fear what brought it on. Phobics may then start reacting in anticipation of giving blood, for instance, dooming them to the same loop of tension-fear-faint. The genetic component, scientists argue, comes only in the fainting part.