14 DAY TRIAL // Every wave of disgust is not a shallow issue, but a biological problem more serious than... losing a meal. "The reason we experience disgust today is that the response protected our ancestors. The emotion allowed our ancestors to survive long enough to produce offspring, who in turn passed the same sensitivities on to us," said Dan Fessler, associate professor of anthropology and director of UCLA's Center for Behavior, Evolution, and Culture.
He explained the methods through which disgust may have been useful for our ancestors: to guard them during such risky situations as pregnancy and to increase their fertility.
But what worked for our ancestors may not be so useful today, provoking xenophobia, sexual prejudices and other irrational reactions. "We often respond to today's world with yesterday's adaptations. That's why, for instance, we're more afraid of snakes than cars, even though we're much more likely to die today as a result of an encounter with a car than a reptile," Fessler said.
In one approach, Fessler asked 400 subjects to imagine 20 different transplant operations and to give them a mark connected to the disgust levels they provoked. Half of the transplant organs were appendages (like tongues and genitalia) exposed to direct contact with the environment and are therefore more prone to infection while the other half were inner organs (like the spleen and heart).
"If disgust protected our ancestors from pathogens, the emotion would have had the most utility in protecting parts of the body that interact most with the environment such as appendages. Our ancestors would not have enjoyed the same advantage from disgust reactions with regard to protecting internal organs. So they benefited from focusing disgust reactions on the parts of the body that are on the outside and interface with the world around us." Fessler said.
Indeed, subjects perceived the idea of transplanting appendages more disgusting than for internal organs. Tongues, genitalia and anuses were the most disgusting, while hips, kidneys and arteries scored the lowest marks. "The disgust we feel when we consider individual body parts reflects an adaptive goal of avoiding the transfer of pathogens," Fessler said.
A pregnant woman's queasiness during the first trimester of pregnancy, when the high hormonal level decreases the capacity of her immune system, could have been a way to fight the "foreign" genetic material taking shape in the womb.
As an infection is highly dangerous for the fetus now, Fessler supposed that natural selection could have chosen pregnant women with a behavioral response that balanced for their lowered immune system.
Fessler put 496 healthy pregnant women aged 18 to 50 to score 32 potentially fulsome scenarios, like "a 30-year-old man who seeks sexual relationships with 80-year-old women," "walking barefoot on concrete and step(ping) on an earthworm," "someone accidentally stick(ing) a fish hook through his finger" and "maggots on a piece of meat in an outdoor garbage pail."
Indeed, women in their first trimester were much more sensitive on the disgust scale than those in the second and third trimesters. But for the morning sickness, the scenarios linked to food, like the maggot example, scored the highest. "A lot of the diseases that are most dangerous are food-borne, but our ancestors could not afford to be picky all the time about what they ate. Natural selection may have helped compensate for the greater susceptibility to disease during this risky point in pregnancy by increasing the urge to be picky about food, however much additional foraging that required. That the sensitivity seems to lift as the risk of disease and infection diminish is consistent with the view of disgust as protection against pathogens."
At least some xenophobia could be linked to the same vulnerable trimester.
206 healthy American pregnant women aged 18 to 42 were put to read two essays, one obviously written by a foreigner critical of the US and another by a patriotic American citizen. The subjects were asked to rate their interest in meeting and working with the authors. Pregnant women in the first trimester were much more reluctant to meeting the foreigner than those in their second and third trimesters were. "Since the need for assistance from any other human being increases with pregnancy, the response doesn't make sense unless you consider outsiders as carriers of disease and infection," he said.
"We suspect that, around the world, cultures have discovered that an easy way to elicit prejudice toward outsiders is to associate them with illness. Because emotional reactions that protect against disease are elevated during the first trimester, xenophobia comes along for the ride and is similarly increased early in pregnancy."
Women also experienced higher disgust towards certain types of sexual behavior during the time in their ovulation, when they are more likely to become pregnant, in another approach made on 307 women aged 18 to 45.
The disgusting scenarios included sex between couples separated by great age spans, incest and bestiality. "Since women have been shown to be the most interested in sex and new experiences when they are the most fertile, their disgust reactions toward unusual forms of sexual behavior during ovulation don't make sense except when considered in the context of reproductive fitness," Fessler said. "These are sexual activities that either would not result in conception or - in the case of incest and sex with older people - were less likely to result in conception of healthy children, so women who were more disgusted by them during ovulation would be more likely to reproduce and to have healthy children."