We overwash our bodies because bad odors are uniquely disgusting

The skin is the New York City of organs. Some of the locals, like the natural oils, are homegrown; others, such as the good bacteria, immigrated in. The skin is dirty, but it needs to be. Otherwise, the inhabitants couldn’t coexist. Sterilizing the skin with disinfectants and heavy soap strips away the well-balanced community, leaving the skin irritated, cracked, and even prone to infection and cancer. And yet, people continue to overwash. According to the comments on a recent Big Think Science Facebook post, people are concerned that adjusting their bathing regimen will leave them stinky.

It isn’t surprising that no one wants to smell bad. But it is interesting that nearly everyone is willing to put their skin health at risk to avoid emitting an unpalatable odor. Such uniformity suggests that this isn’t just a matter of preference, but instead, a biologically-driven behavior. 

The behavioral immune system and disgust

If a person smells like a bag of egg salad left in the sun during an Alabama summer, their smell is a little disgusting, and we would like to stay away from them, regardless of how perfect and beautiful their skin might be. In other words, bad odors can trigger disgust, which can powerfully influence our behavior.

Disgust is activated by the Behavioral Immune System (BIS). In the landmark paper “The Behavioral Immune System (and Why It Matters),” Mark Schaller and Justin Park — experts in disgust and evolutionary psychology — describe the BIS as “behaviorally analogous to the classic immune system”: both systems detect cues that indicate the presence of potential disease-causing threats, and both systems trigger a response to protect the host from those threats. Whereas the classic immune system possesses an arsenal of cells and proteins to protect the host, the BIS deploys disease-relevant emotional and cognitive responses. Indeed, people who more easily feel disgusted are less often infected, suggesting that avoidance behaviors are effective. 

However, BIS responds to an overly general set of cues, which can result in aversive responses to things (including people) that pose no actual threat of pathogen infection. Thus, we shun and stigmatize people who not only potentially carry biological disease, but also those who violate the social conventions that we perceive as threatening. In other words, we might be disgusted by a person with a foul odor because they potentially carry infectious disease but also because they smell different than society expects.

So, if disgust is supposed to protect us from disease, and if overwashing can cause disease, why aren’t we disgusted by overwashing? One reason is that skin diseases are often visual in nature, and visually-induced disgust is not nearly as powerful as odor-induced disgust when it comes to influencing our behavior, according to previous research. And that is because smell is processed differently than other senses. 

Smell has a direct connection to emotions

Scent molecules are initially detected by receptor neurons in the nose. These neurons send information about these encounters first to the olfactory bulb, a brain structure about the size of a marble located above the nasal cavity. Unlike our other senses, olfactory nerves do not proceed to the brain’s thalamus, the region responsible for creating a conscious understanding of sensory information.

Instead, information from the olfactory bulb takes a direct route to a tiny area of the brain called the amygdala, where emotions are processed, and then to the adjoining hippocampus, where learning and memory formation take place. This results in an intimate connection between scents, memories, and emotions (such as disgust). Because of this direct route, we have an immediate, visceral reaction to odors.

This process is helpful and evolutionarily advantageous. For example, when the smell of rancid food hits the olfactory bulb, the brain immediately triggers a surge of serotonin in the gastrointestinal tract. This induces the gag reflex, which is the body’s attempt to purge potentially contaminated food.

Such a visceral reaction, however, can also cause problems, especially in a society that is over-obsessed with cleanliness. It is good to rinse off sweat and dirt, but a little body odor is normal and carries no risk of infection. Our disgust over body odor is largely conditioned into us, in no small part by the commercial health and beauty industry, which has successfully convinced us that natural body odor is a sign of poor health. That is, the reason we overwash, despite the risk to our skin health, is that we are more worried about the consequences to our social health than to our biological health.