Sick of How You Smell?
Scientists show that differences in our vulnerability to disease affect how much we prefer our own body odor and the body odor of others.
Our vulnerability to illness affects how we perceive our own body odor and the body odor of others, say scientists from the University of Warwick (UK). Variable body odor preferences may be an evolved mechanism that slows the spread of disease.
Body odor, attraction, and disease may not seem like natural bedfellows. But finding a mate and resisting infection are two of the most important challenges faced by any organism, and humans are no exception. If we succumb to a disease, we won’t be able to reproduce. If we can’t reproduce, we’re an evolutionary dead end. It really is survival of the fittest (or perhaps survival of the healthiest).
We resist disease by means of two defence mechanisms. The first is the classical immune system, typified by the white blood cells that engulf germs, and the tiny platelets that seal up and repair injuries to blood vessels. The second defence mechanism is the behavioral immune system, which is designed to keep parasites and plagues at arm’s length. When we are disgusted by the feel of creepy crawlies on our skin, the sight of an infected wound, or the stench of rotten meat, our behavioral immune system is ordering us to maintain a safe distance.
Now, some of us have strong classical immune systems, and others have weak classical immune systems. If you’re the sort who comes down with a cold on the first day of winter, or a dose of food poisoning after catching a whiff of a day-old profiterole, you’re better off with a hair-trigger behavioural immune system. The more sensitive you are to disgusting stimuli, the less likely you are to get infected and be forced to muster your battle-weary white blood cells against hordes of invading bacteria.
One source of illness is other people, and our behavioural immune system fires up whenever we encounter someone with a snotty nose or a face covered with measles. But an unfortunate consequence of our behavioural immune system is that it can also drive outgroup bias — one form of which is racism. This is because millennia of evolution has taught us that we’re more likely to catch the lurgy from a visitor two villages over than from a member of our own tribe. Just ask the natives of America, who were decimated by smallpox and other Old World diseases brought by the Conquistadors. The Spaniards and Portuguese were immune to diseases that the Inca and the Aztecs had never encountered and against which their classical immune systems were defenceless.
Another reason for this reticence to mix with strangers is that we all have slightly different classical immune systems, with genes that are good at combating some infections but weak at dealing with others. We are more likely to share genes with people whose genetic history is similar to our own, especially family members. And this is where body odor comes in: the same genes that control immune function also influence our natural scent. It’s possible to sniff out our genetic doppelgängers!
Naomi Muggleton and Corey Fincher of the University of Warwick decided to test whether people with a keen sense of disgust are more attracted to body odors that are reminiscent of their own. If so, this would suggest that those who are more prone to falling ill are drawn to their genetic doubles, from whom they are less likely to catch a cold.
Muggleton and Fincher had ~50 men and women complete the Perceived Vulnerability to Disease questionnaire (or PVD for short), which requires respondents to agree or disagree with various statements about illness and infection. There are two types of statement in the PVD. One tests whether a respondent thinks they have a history of infection, for example: “ In general, I am very susceptible to colds, flu and other infectious diseases”. The second type of statement tests the respondent’s aversion to germs, for example: “ I prefer to wash my hands pretty soon after shaking someone’s hand.”
After completing the PVD, Muggleton and Fincher’s participants opened up eight glass jars and sniffed the contents. Inside each jar was a pad of gauze. These pads of gauze had spent 12 hours taped under a volunteer’s armpit. It’s safe to say, they stank to high heaven. Participants inhaled deeply and rated the eight pads for attractiveness, desirability, and pleasantness.
All the best psychology experiments involve a smidgen of deception, and this experiment is no exception. Although seven of the pads were provided by strangers, the eighth pad belonged to the participant him- or herself. A couple of weeks earlier all the participants had worn gauze pads under their armpits overnight, returned them to Muggleton and Fincher in sealed bags, and were led to believe that these pads were to be used in a different research project. Not so! The donors were reunited with their pads the next time they visited the lab.
For each participant, the researchers calculated a Self Preference Index by subtracting ratings of the strangers’ pads from the rating the participant gave to their own pad. A high score meant that a participant preferred their own smell to that of strangers, and a low score that strangers’ body odors were more appealing.
Muggleton and Fincher found that, among women, preference for own odor was stronger if scores on the germ aversion section of the PVD were high. That is, women who were particularly worried about coming into contact with germs tended to prefer body odors similar to their own. Women who are grossed out by poor hygiene and germ-infested handshakes may seek solace in their in-group, where people smell the same and share similar immune system genes.
However, among men the relationship between germ aversion and preference for self-similar smell was in the opposite direction. Men who were not worried about germs expressed a preference for self-similar odors.
Why should men and women differ? Both men and women can fall ill, both men and women feel disgust, and both men and women are motivated to reproduce. Surely their behavioural immune systems should function identically.
Perhaps not. Males and females evolve under distinct pressures and evolution has shaped us to respond differently to similar situations. One of the most obvious differences between the behavior of males and females is that females do most of the child-rearing (the paternal seahorse is such a striking case because nurturing fathers are a rarity in nature). As Muggleton and Fincher point out:
…in ancestral environments, females may not have the luxury of retreating
from dependent offspring. For men, we speculate that a tradeoff between kin- and self-protection can occur, such that the ancestral male eschews support, to protect close kin and other ingroup members from infection.
So, when a feller contracts man-flu, and retreats to his den for a Lemsip and a reinvigorating X-Box session, leaving his partner to tend to the kids, it’s probably because he cares.
And because his family smells awful.