Would you use a shopping cart that had a crumpled grocery list sitting at the bottom of it? Probably not. Why does a wrinkled piece of paper make a shopping cart disgusting? The cart that you ended up using could have been touched by hundreds of people since its last cleaning and the cart with the shopping list might have been washed that morning.
But a dog-eared note with a stranger's scrawl is an unmistakable sign that other people have used and touched that cart, and this makes it repellent. Indicators of other people, whether past or present, are disgusting to us because they implicitly bring up thoughts of contamination and disease, regardless of cleanliness. A primary objective of disgust is to keep the outside away from the inside. And just what or whom we find repellent is key when it comes to protecting ourselves.
Consider the results of an online survey with nearly 31,000 respondents: People voted the mailman as the person they would least like to share a toothbrush with and their spouse as the most acceptable toothbrush-swapper. The other possible choices were the boss, a best friend, the weatherman, and a sibling—best friend and sibling followed closely behind spouse.
Sharing bodily fluids with unfamiliar people is considered more disgusting because strangers pose more of a health threat since we don't know where they've been or what they've been doing. The germs they carry are mysterious in quality and quantity and therefore potentially dangerous. By contrast, you already share so much with family members and other intimates that their germs are less likely to harm you. You have either already developed an immune response to these germs or you carry these bugs as well.
However, how much we like a person and how attractive they are moderate how agreeable we are toward toothbrush-swapping, independent of common contact. One's boss comes in second (after the mailman) as least acceptable to share a toothbrush with, whereas the weatherman is three times more acceptable than the boss. Presumably you have more physical contact with your boss than with a TV weather reporter, but bosses are often disliked and TV anchors are usually well-dressed and good-looking. Our attraction toward someone can override all qualms about body fluids.
Several studies have found a link between social rejection, disgust, and illness. People who believe they are at an increased risk of getting sick—either because they are hypochondriacs or they've been primed with images of germs—react more negatively to foreigners and overweight people; also, they are less likely to be friends with those who have physical disabilities. Just being exposed to images or information about illness leads people to become less agreeable, less sociable, and to automatically use gestures that signify avoidance.
Frankly, it does make sense that we run from the unknown. Think of the guy who coughs into his hand and then visits the ATM machine; a study in China revealed that each key on an outdoor ATM had some 1,600 germs. In fact, people who are slightly obsessive compulsive and contaminant-sensitive are actually healthier; those who avoid objects touched by strangers report fewer colds, stomach bugs, and other infectious ailments.
Vigilance about germs is clearly adaptive to humans, but we do not innately avoid cooties. In truth, disgust is the last emotion children acquire. Happiness comes first, followed by sadness, and both emotions appear very early on. Despite the fact that a baby will grimace and spit out things that taste bitter from birth, she won't experience any other form of disgust until she is at least 3 years old. And it's not until she's at least 5 that she'll have an inkling that the look on your face means that you're disgusted by the dead skunk she's just brought home. Indeed, she'll most likely think that you're angry with her.
Our early obliviousness to disgust may be adaptive because mucking around in the dirt strengthens our immunity. Exposure to benign bacteria stimulates the immune system so that it is better able to fight bad bacteria. The old-fashioned approach of letting toddlers crawl around on the floor actually results in better overall health when children are older.
But we don't get it all at once; we have to learn what is disgusting. Disgust is the most advanced human emotion, and it requires reasoning, thought, and deduction. Humans are the only animal with a brain advanced enough to process this complexity.
Although disgust helps us stay safe by keeping us away from our number one predator, pathogens, it doesn't always get it right. For example, we're not at all disgusted by money, and cash is the most pathogen-ridden thing around us, while disgust at benign creatures like earthworms can lead us to avoid the many enjoyable outdoor activities that keep us healthy.
Rachel Herz, Ph.D., is an expert on the psychology of smell. This is adapted from her book, That's Disgusting: Unraveling the Mysteries of Repulsion. Read her PT blog: Smell Life.
A Heavy Burden
The overweight are correct that they are discriminated against. Negativity toward the overweight is so pervasive that it is even contagious. In one study, not only was an obese job candidate judged less competent and worthy than a person of average weight, but a man of average weight sitting next to a professionally dressed obese woman was also judged as a less capable and desirable job candidate than when he was seated beside a similarly dressed woman of average weight. Ironically, the bystanding female candidate was the same person; she wore a fat suit in half of the study. The negative contagion of being near an overweight person even persisted when it was made clear that the physical proximity was accidental.
Strangers in a Strange Land
Immigrants and ethnic groups are often seen as disease-spreading foreigners. For more than a century, the U.S. has seen waves of foreigners and these "aliens" have been greeted with less than open arms. Instead, they were the subject of rumors: They posed a health threat to "rightful" citizens. Following World War I, North America adopted the "open air school movement." This public health initiative originated in Berlin in 1904, and it was so influential that it had a major impact on school architecture. The purpose of the open air movement: to clear and clean the air of tuberculosis germs that immigrant schoolchildren, predominantly Italians and Jews, were suspected of spreading. Classrooms were built with walls of enormous windows that could be fully opened and kept open at all times, even during snowstorms and frigid winters.