The limits of the proposed 'behavioral immune system'

Many evolutionary psychologists argue that humans have a sophisticated ‘behavioral immune system’. We are, so the story goes, endowed with a suite of sophisticated, evolved psychological mechanisms that enable us to effectively detect and avoid disease causing parasites and pathogens. Unfortunately, work on the ‘behavioral immune system’ is plagued by one key methodological issue, and ethnographic evidence points to a more complex picture.

The core problem with existing research on the ‘behavioral immune system’ is that it is largely restricted to surveying people in modern nation-state societies. This is a big problem for two reasons. One, people in modern nation states tend to be aware of the germ theory of disease. That is, for cultural-historical reasons, they have scientifically informed knowledge of how disease is transmitted. This means disease-avoidance behavior can follow for reasons that are socially learned, and not the result of evolved mechanisms to avoid pathogens per se.

Consider for example an ethnographic account I wrote of previously, which demonstrates the behavioral contrast between someone with culturally learned knowledge of how pathogens spread compared to beliefs in a society with a different understanding of disease: “In The Afterlife is Where We Come From (2004), anthropologist Alma Gottlieb describes her fieldwork among the Beng farmers of West Africa. In this book, Gottlieb recounts her failure to persuade Beng villagers to boil their drinking water;

During our stays in Beng villages, Philip and I have always either boiled or filtered our own drinking water. To our dismay, our neighbors often derided our laborious efforts. One day we thought to explain our mysterious actions. The village had been experiencing an especially crippling outbreak of Guinea worm. After reading about the disease, Philip and I were convinced that polluted drinking water was the cause of our neighbors’ misery. We urged our friends to boil their water as protection against future infestation. But even our closest and most open-minded friends dismissed our suggestion with casual laughter.

“Can you see the worms in our water?” our friend Yacouba challenged us. We admitted we couldn’t.

“There’s nothing wrong with the water,” he insisted. “Anyway, even if the Guinea worms come to us through the water, they’re put there by witches.” Yacouba added emphatically, “Boiling the water wouldn’t stop the witches.” (Gottlieb, 189).”

Second, people in state societies often have access to culturally developed antiseptic systems and substances, facilitating greater pathogen avoidance than was often possible historically. This is important for an evolutionary model—if we want to investigate a pan-human ‘behavioral immune system’, and consider how it evolved, we should be looking at behavior in places where people don’t have historically recent knowledge of the germ theory of disease or access to antiseptic inventions, where they have to grapple with pathogens in contexts more similar to our evolutionary history than the WEIRD environments of today.

Let us consider a few domains where there is evidence across traditional societies that complicates that ‘behavioral immune system’ framework.

First, putrid or rotting meat. Anthropologist Frank Marlowe writes of the Hadza hunter-gatherers of East Africa that, “the Hadza often eat very rotten week-old meat they scavenge from carnivores,” adding that, “The scavenged (and sometimes very rotten) meat also gives them stomachaches, even after they cook it, but not often enough to keep them from eating it.”

Anthropologist C.R. Hallpike touches on this a great deal in his book Ship of Fools, writing that, “In Papua New Guinea the people are even less fastidious about their pork when it is stinking, even cutting it up under water when the smell is too bad. And yes, sometimes they died from it. Biologists often tell us that we are protected by the vomiting reflex from eating meat that has begun to smell as it goes bad, but no one seems to have told primitive peoples about this.”

Missionary Angel Turrado Moreno writes among the Warao of the Amazon of the, “Fish and meat, half raw, spoiled, and even rotten, which they eat without the slightest scruple at any hour of the day or night.” Among the Yaghan of Tierra del Fuego beached whales were often feasted upon, even when putrid or rotting, and digestive disorders caused by spoiled meat were said to be common.

Now, to some degree this can fit within the ‘behavioral immune system’ framework, with the important acknowledgement that tradeoffs matter a great deal. In their paper on ‘Human pathogen avoidance adaptations’, evolutionary psychologists Joshua M. Tybur and Debra Lieberman write that, “Individuals should be more willing to accept the potential costs of infection via food if nutrients are more needed.” Under this logic, the willingness to consume rotting or putrid meat reflects necessity borne from a scarcity or shortage of nutrients. This seems reasonable, although I think this is something that would still require greater investigation.

It is clear that ecological and cultural factors play an important role in disease avoidance behaviors to a degree that indicates the limitations of assuming humans have a powerful and predictably expressed ‘behavioral immune system’. For example, opportunities for good hygiene are often quite rare across many small-scale societies, and even if they are ecologically possible cultural beliefs and knowledge determine to a large degree what hygiene practices do or do not develop. Hallpike writes that, “Where water is scarce and is arduous to fetch, drinking and cooking take priority over hygiene by a wide margin, and I never saw the Konso [of Ethiopia] (or the Tauade [of New Guinea], for that matter) wash their hands before eating.”

Anthropologist Ran Singh Mann writes among the Onge Andaman Islanders “The raw material for food is not properly cleaned. It is boiled in dirty water containing mud and dust. The utensil in which the food is cooked is hardly cleaned. It is used over and over again without being cleaned. Layers of dirt keep on accumulating on the utensils used for cooking and eating purposes. The dogs also eat the left-overs in the same containers.”

Beyond limited hygiene practices, extensive and visceral interactions with dead bodies offers one of the most powerful complications for the ‘behavioral immune system’ framework. Hallpike quotes a patrol officer writing in 1945 that among the Tauade, ““A recently bereaved widow had an arm bone, several rib bones and the complete hand of the late departed hanging on a string around her neck. She did not appear to mind the offensive odour.”” Among the Inuit on Kodiak Island, “Nobody was afraid to touch a corpse,” and burials would not be conducted until the fourth day after death, “Until then the corpse is never left alone,” and is given a place in the main room of the house. Rituals involving corpses where also common among whale hunters in the region: “The whale hunters preserved the bodies of renowned men in caves, where they assembled prior to the hunt, carried the corpses to nearby streams, laid them in the water, and drank from this water.”

Andaman Island girl wearing her sister’s skull. From Radcliffe-Brown (1922).

In Australia, James Dawson writes that, “Before the minds of the aborigines were poisoned by the superstitions of the white people, they had not the slightest dread of the dead body of a friend, nor had they any repugnance to remain beside it." And, as I noted in a previous article touching on this topic, anthropologist Bruce Knauft described mourning rituals among the Gebusi of New Guinea,

The following morning, Dugawe’s body was grossly bloated. His swollen limbs oozed corpse fluid, and his peeling skin exposed putrid yellowgreen flesh. His belly and even his genitals had swelled with the gases of decomposition. The stench was unforgettable; it burned up my nose, down my throat, and into my brain. Equally powerful were the actions of Dugawe’s female kin. With unearthly sobs, they draped themselves physically over the corpse, lovingly massaged its slime, and drew back its skin. They rubbed their arms and legs with the ooze of the body. Corpse fluid on one’s skin is a tangible sign of grief, of physical as well as emotional connection to the deceased—making one’s own body like the corpse. Seeing this, Dugawe’s departing soul was said to know how much they cared for him and ease his anger at having died, at least a little (Knauft, 51).

See also mummification practices among the Anga of New Guinea. And kuru, the prion disease that spread through ritual cannibalism among the Fore of New Guinea.

Blood rituals are also not uncommon. As I previously noted, “Men of the Arunta foragers of Australia would drink some of each other’s blood under the belief that it would make them stronger and prevent treachery. Anthropologists Francis Gillen and Walter Spencer wrote that, “If [a man] refused to drink the blood, then, as actually happened in one case known to us, his mouth would be forced open and blood poured into it, which would have just the same binding influence as if the drinking had been a voluntary one.” Gillen and Spencer also discuss practices of treating sickness with blood, adding that,

When a woman is very ill and weak, one of her male Umba, to whom she is Mia alkulla – that is, he is the son of one of her younger sisters – may volunteer to strengthen her with his blood, in which case all the women and children are sent away from her. The man draws a quantity of blood from [his] sub-incised urethra, and she drinks part of it, while he rubs the remainder over her body, adding afterwards a coating of red ochre and grease. [italics added]”

Warriors in some societies may smear the blood of enemies on their bodies, or consume body parts to gain power. Among the Ingalik hunter-gatherers of Alaska, “a warrior may eat the eye of a respected opponent whom he has killed. When such a thing is done, it is for the purpose of gaining an enemy's power.”

The ritual use of animal excreta also complicates the ‘behavioral immune system’ framework. Infants would be ritually bathed with female camel urine among the Bedouin pastoralists of the Middle East [camel urine has also been known to be traditionally consumed in many parts of the region], while male Nuer pastoralists of Sudan would ritually rub the ashes of cattle dung on their bodies. Among the Fulani of Niger, “The feces (belade) from the cow are used for healing purposes. Women sometimes cook the belade and rub it into their hair to make it look darker. Cow urine is considered to have healing capabilities as well, and people who have been ill for a long time wash with the urine.” Cattle dung is also commonly used among many pastoralists as fuel or building material, although this has a clearer and more direct function than its ritual usage.

To sum up, I am not arguing humans never avoid pathogens, or that they are completely unconcerned with doing so. I think universally across cultures humans do share some sort of disgust ‘instinct’, although what facilitates a disgust reaction is far more culturally and historically contingent than many evolutionary psychologists would likely expect, in ways that complicate an emphasis on our supposed ‘behavioral immune system’.