Most phobias—of cockroaches, spiders, heights or clowns—don’t induce the wooziness typical of blood phobia. Some of the people who fear of blood will pass out at the sight of the stuff. Popular Science explains why this is so strange:
Despite it being relatively common—3 to 4 percent of people suffer from blood phobia or a related disorder—the symptoms of it are totally different from most phobias: phobics’ blood pressure and heart rate will rise then drop when they see blood, as opposed to the just-heart-racing caused by most fears.
Not much research has been conducted to explain why this happens, John Sanford of Stanford Medicine writes. But those studies that have examined the topic have yielded mixed results. Some say that fainting at the sight of blood may be the human equivalent of playing opossum—pretending to be dead so that a dangerous predator will lose interest. Others think that the physiological reaction some experience at the sight of blood may be an evolutionary adaptation. If a caveman got stabbed in the foot while out on a hunting trip, Sanford explains, he may have a better chance of surviving if his blood pressure drops, helping him to avoid bleeding to death.
Yet blood phobia presumably would not — at least in modern times — provide much in the way of selective advantage. Emergency medical responders generally can reach you quickly and stanch bleeding. And if you faint, you can sustain a worse injury by falling.
So besides being useful for dramatic effect in the movies, it seems blood phobia—perhaps like the appendix or wisdom teeth—is an evolutionary throwback that has largely outlived its usefulness. Now, if those of us who suffer from the phobia could only convince our pounding hearts of this logic.