Why Are We So Grossed out by Our Own Bodily Fluids?

Here's the evolutionary reason why we're not disgusted by blood, sweat and saliva until after it leaves our body. Hint: It may just save your life.

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The human body is filled to the brim with gallons of oozing substances that serve many varied and vital functions. This is something we’re all aware of on some level, and most of the time we’re perfectly happy to go about our day with all these liquids gurgling away inside us. And yet, should one of those fluids exit our bodies—say, we cut ourselves shaving, or sneeze when not in range of a tissue—we’re suddenly revolted. Certainly, the thought of putting that substance back inside your body would be unthinkable, despite it having been there mere seconds before.

It seems a strange reaction, considering how important all of these fluids are. Your blood, for example, does everything from transporting oxygen to your brain to regulating your temperature—big jobs that require a Costco-sized serving of the stuff (a six-foot, 200 lb man might have as much as 7.2 liters of blood shooting through his veins at any given moment). Saliva—produced in quantities as great as two liters a day—protects your mouth and teeth from bacteria and decay. Mucus—that scourge of parents with small, bubble-nostriled children everywhere—coats your airways to stop foreign particles and chemicals from getting into your lungs, with your body producing up to a liter of the gunk a day if you’re sick. This production line of life-saving liquids is a biological marvel, frankly, so why are we so disgusted by it?

According to Dr. Martin M. Antony, a professor of psychology at Ryerson University in Toronto, Canada, the grossed out response to our various secretions is the result of a series of learned responses we picked up during our evolution, as an innate defensive mechanism to ensure our safety and survival. Essentially, it’s all part of your body’s way of avoiding sickness.

“Often, the things that trigger a disgust reaction are things that can carry disease,” Dr. Antony says. “Think about animals that lead to a disgust reaction, like mice or bugs or things like that. Evolutionarily, we’ve developed this mechanism when we react to something that can potentially make us sick—whether it’s mold on a raspberry or blood, we have a disgusted reaction.” This reaction seems to have been learned very early on by our ancestors—certainly millennia before humans actually discovered the existence of bacteria and germs in the 18th Century.

The body can exhibit more extreme reactions to the sight of bleeding, too, such as fainting, something that happens due to a natural defence mechanism that harkens back to our earliest ancestors. If a catastrophic loss of blood occurred—say, if a limb was hacked off by a rival—the body would react with a sudden drop in blood pressure that prevented too much blood from being pumped out of the body. The resulting faint also acted as a form of self defense in its own right—an attacker might be less inclined to finish you off if he saw you drop down, assuming you were already dead.

None of this is particularly helpful today, of course, especially when it’s the sight of someone else’s blood that causes you to pass out. And it’s not uncommon: Jon Hershfield, director of the OCD and Anxiety Center of Greater Baltimore in Maryland, works primarily with patients who suffer Obsessive Compulsive Disorder, where an extreme fear of bodily fluids is, he says, one of the more common manifestations.

“There’s a fear of something bad happening if they come into contact with a bodily fluid, and if it’s their own, they wonder if there’s something wrong with their blood. What if they get it on someone and they get sick?” Hershfield says. “It’s an obsession about making sure you’re not getting sick if you come into contact with them.”

Antony confirms that he’s witnessed several cases where this fear, particularly regarding blood, can have a profound effect on a person’s life, including promising medical students dropping out of school because they couldn’t deal with the sight of blood. The good news for those with such a chronic phobia is that the condition is very treatable. “When you encourage them to do the things they are afraid of, the fear typically comes down,” Antony says. “It’s quite easy to treat, actually—often in anywhere from one to three [therapy] sessions. It’s a pretty quick treatment compared to the other problems we see.”

With that in mind, the next time some viscous liquid seeps from your skin and the portion of your brain that registers disgust starts to tingle, just remember that both your fluids and your disgust are there for a reason: To keep you and that fleshy sack of sloshing gunk that you call your body alive.