Image: Friends, Warner Bros.
Most people find the sight of blood or a hypodermic needle enough to cause some discomfort, but why do some people faint when faced with them? If you’re someone who finds yourself sweating about your upcoming flu jab, you might have your prehistoric ancestors to thank.
Phobias are part of the anxiety disorder family. They are thought to arise because of a learned response to a stimulus following a traumatic event (being bitten by a dog might lead to a fear of dogs), or because of intrinsic adaptive mechanisms that promote survival, which might underlie a fear of spiders or heights.
Most people are familiar with the experience of fear. It may be fear of an upcoming presentation, or of a mouse running across your kitchen floor. Fear is a basic emotion central to the experience of threat, where an animal either fights the threat or runs away.
A phobia, on the other hand, is an intense, pervasive and debilitating fear of something that might seem entirely harmless to others.
The blood-injection-injury phobia.
The blood-injection-injury phobia is a fairly common phenomenon. It is experienced by approximately 3% of the population. The phobia can be triggered by the sight of blood, by sustaining an injury, receiving an injection, or some other type of medical procedure.
All humans have a natural tendency to be squeamish in these situations, but for some people the response is more extreme. They experience a temporary spike in heart rate and blood pressure, followed by a dramatic drop. This results in skin pallour, sweating, nausea and fainting.
Watch: The Mamamia team tries (and fails) to pronounce some complex phobia names. (Post continues after video.)
This fainting response is unique to the blood-injection-injury phobia, in contrast with the usual acceleration of heart rate and elevation in blood pressure in all other phobias.
So why does a fear of blood or needles leave a person weak at the knees, while confrontation with a spider or a Ferris wheel leaves the body armed and ready to fight or run away?
The earliest “adaptationist” hypothesis to explain the blood-injection-injury phobia suggests fainting at the sight of blood increases the chance of survival, because a dramatic drop in blood pressure minimises blood loss in the case of injury. However, this does not explain why people faint when faced with needles or minor injuries, where little or no blood loss is involved.
The second evolutionary hypothesis posits that blood-induced fainting in mammals is controlled by the same physiological mechanism that regulates disgust. It suggests that, in some people, the sight of their own (or another’s) blood might induce a disgust response.
However, disgust (and associated nausea and vomiting) is thought to have evolved to protect mammals from the risk of disease-laden food. It’s difficult to imagine the adaptive benefit of fainting when confronted with bad chicken; simple avoidance seems like a much better approach to maintaining health and survival.