“Many will swoon when they do look on blood.” (Shakespeare, As You Like It, Act IV, Scene III)
Some people know this phenomenon only from movies, TV shows or books. Others from relatives, friends or even themselves: The terrible weak feeling of fainting that is triggered by the sight of a large amount (or sometimes even just single drops) of blood. Such people are, in most cases, not suitable for donating blood, not to mention, work in emergency rooms in hospitals.
But where does this strong reaction come from? Is it even good for anything?
First of all, we are talking of the so called blood phobia, also known as hemophobia. It is part of a whole group of blood-injection-injury phobias (BII), as categorized by the Diagnostic and Statistical Manual of Mental Disorders (DSM) .
The general consensus behind the cause of exaggerated blood phobia, which results in vasovagal responses, is that they originate from the psychological traits of an individual rather than from their genetic heritage. It seems, for example, sometimes to be caused by childhood traumata . On the other hand, twin studies suggest that there might also be certain genetic predispositions which are common for phobias in general .
Anyways, are there any explanations? Indeed, there are three more or less fascinating ideas that could hold the key:
(1) The danger theory: Seeing blood is an alarm signal. So when we start feeling weak, we automatically seek for a safe place to rest and/or hide. This would of course only make sense, if the process of fainting takes some time, allowing us to act.
(2) The “play dead” theory: During stone-age, some predators were not interested in paralyzed preys. They would actually wait for a person to flee, only to follow them. Good for the people with hemophobia during those ancient hunts!
(3) The self-healing theory: The blood pressure decreases during fainting. An injured person could thereby slow down the blood loss and instead support the blood coagulation.
Whatever the true origin might be, nowadays the fear of blood is nothing more than annoying. But luckily, as with any phobia, blood phobia can be cured .
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 Sanford, J. (2013), Stanford Medicine, Spring 13. http://sm.stanford.edu/archive/stanmed/2013spring/article6.html