The evolutionary logic behind disgust is seemingly straightforward, but it does have quirks. Consider the following situations:
• Drinking a glass of spoiled milk.
• Stepping in vomit.
• Being sneezed on.
• Getting a fishing hook stuck in your finger.
• Touching the ashes of a cremated body.
• Using your mouth to inflate a new, unlubricated condom.
• Eating soup stirred with a sterilized flyswatter.
• Eating fudge shaped like dog feces.
Each of these situations typically elicits some degree of disgust, but the first four are qualitatively different from the last four. The first four entail a genuine threat of infection, whereas the last four do not. The last four are disgusting only by association — either perceptual association with a disease-ridden object (the feces-shaped fudge), functional association (the new condom, the sterilized flyswatter), or historical association (the ashes of a cremated body). The boundaries of disgust are readily extended to encompass substances that pose no real threat, because it’s ultimately better to avoid something disease-neutral than to touch or ingest something disease-ridden. Avoiding a disease-neutral object is inconsequential from an evolutionary point of view, but exposure to a disease-ridden object can be a game stopper.
Our hypersensitivity toward the threat of contagion can be seen not just in our contemplation of hypothetical situations but also in our actual behavior. Adults offered a piece of fudge in the shape of dog feces typically refuse to eat it, even though they know perfectly well that the fudge is not contaminated. Adults offered juice stirred with a sterilized cockroach typically refuse to drink it, even though they know perfectly well that the juice has not been contaminated. Likewise, most adults refuse to hold a disc of plastic vomit between their teeth, eat soup out of a brand new bedpan, or eat sugar out of a bowl labeled “cyanide,” even when they themselves affixed the label to the bowl.
Perhaps least rational of all, most adults refuse to eat soup from a bowl they have spit into or drink water from a glass they have spit into, even though they know that each bite of soup or each sip of water will mix with saliva the moment it hits their tongues. None of these activities are physically harmful, but they are still psychologically harmful.
Our tendency to confer disgust by association would be nothing more than a quaint feature of human cognition if it weren’t for our failure to associate disgust with many things that should be disgusting. For centuries, infectious diseases like cholera and smallpox spread like wildfire because humans were not disgusted by cholera-infested water or smallpox-infested blankets. These disease-ridden objects betrayed no sign of their deadly nature, so they spread disease more effectively than basic human-to-human contact. Even today, highly avoidable diseases like chlamydia and HIV still plague humanity because the acts that spread them are associated with pleasure rather than repulsion.
Our evolved capacity to avoid pathogens and parasites—disgust—is thus largely ill informed. On one hand, it’s triggered by sights and sounds merely associated with disease, while on the other, it’s left untriggered by the pathogens and parasites imbued in everyday objects. What disgusts us is not always a threat, and what threatens us is not always disgusting.
This is an extract from Andrew Shtulman's Scienceblind published by Basic Books