One morning last fall, the evolutionary biologist Randy Thornhill was standing with me in front of the gorilla enclosure at the Albuquerque zoo. He was explaining a new theory about the origins of human culture when Mashudu, a 10-year-old western lowland gorilla, decided to help illustrate a point. In a very deliberate way, Mashudu sauntered over to the deep cement ravine at the front of his enclosure, perched his rear end over the edge, and did his morning business.
Mashudu, I suspected, had just displayed what evolutionary theorists call a “behavioral immune response”—a concept central to Thornhill’s big theory. So I asked him whether I was right about Mashudu. “Pooping downhill is pretty smart,” Thornhill said after some consideration. “He got his waste as far away from him as possible. I think that would probably count as a disease avoidance behavior.”
It might seem strange to fixate on how a gorilla goes about answering the call of nature. But according to Thornhill’s hypothesis, much of what we humans like to think of as politics, morality, and culture is motivated by the same kind of subconscious instinct that likely drove Mashudu to that ledge.
What kind of government do you live under? Who are your sexual partners? How do you treat strangers? All of these questions may mask a more fundamental one: What germs are you warding off?
Anyone with a basic grasp of biology knows that all animals have immune systems that battle pathogens—be they viruses, bacteria, parasites, or fungi—on the cellular level. And it’s also fairly well understood that animals sometimes exhibit outward behaviors that serve to ward off disease. Just around the corner from the fastidious Mashudu, Thornhill and I watched an orangutan named Sarah grooming her six-month-old son Pixel, poring through his hair for parasites. Some species of primate, Thornhill told me, will ostracize sick members of the group to avoid the spread of disease. Cows and other ungulates are known to rotate their movements among pastures in such a way as to avoid the larvae of intestinal worms that hatch in their waste. And in ant societies, only a small number of workers are given the task of hauling away the dead, while sick ants will sometimes leave the nest to die apart from the group.
At the most quotidian level, Thornhill finds it easy to convince people that humans likewise manifest such instinctual behaviors to avoid infection and illness. Some of these habits very much parallel those seen in other creatures. I admitted to Thornhill that I had recently been displaying a bit of grooming behavior myself after the youngest primate in my care came home from preschool itching with head lice. Like Mashudu, we humans remove waste from our living quarters. We ostracize our sick, at least to the extent that we expect those with the flu to stay home from work or school. And similar to the lowly ant, we assign a small number of our fellows the solemn duty of hauling away and disposing of our dead. On examination, everyday life is full of small defensive moves against contamination, some motivated by feelings, like disgust, that arise without conscious reflection. When you open the door of a gas station bathroom only to decide you can hold it for a few more miles, or when you put as much distance as possible between yourself and a person who is coughing and sneezing in a waiting room, you are displaying a behavioral immune response.
But these individual actions are just the tip of the iceberg, according to Thornhill and a growing camp of evolutionary theorists. Our moment-to-moment psychological reactions to the threat of illness, they suggest, have a huge cumulative effect on culture. Not only that—and here’s where Thornhill’s theory really starts to fire the imagination—these deep interactions between local pathogens and human social evolution may explain many of the basic differences we observe between cultures. How does your culture behave toward strangers? What kind of government do you live under? Who are your sexual partners? What values do you share? All of these questions may mask a more fundamental one: What germs are you warding off?
The threat of disease is not uniform around the world. In general, higher, colder, and drier regions have fewer infectious diseases than warmer, wetter climates. To survive, people in this latter sort of terrain must withstand a higher degree of “pathogen stress.” Thornhill and his colleagues theorize that, over time, the pathogen stress endemic to a place tends to steer a culture in distinct ways. Research has long shown that people in tropical climates with high pathogen loads, for example, are more likely to develop a taste for spicy food, because certain compounds in these foods have antimicrobial properties. They are also prone to value physical attractiveness—a signal of health and “immunocompetence,” according to evolutionary theorists—more highly in mates than people living in cooler latitudes do. But the implications don’t stop there. According to the “pathogen stress theory of values,” the evolutionary case that Thornhill and his colleagues have put forward, our behavioral immune systems—our group responses to local disease threats—play a decisive role in shaping our various political systems, religions, and shared moral views.
If they are right, Thornhill and his colleagues may be on their way to unlocking some of the most stubborn mysteries of human behavior. Their theory may help explain why authoritarian governments tend to persist in certain latitudes while democracies rise in others; why some cultures are xenophobic and others are relatively open to strangers; why certain peoples value equality and individuality while others prize hierarchical structures and strict adherence to tradition. What’s more, their work may offer a clear insight into how societies change. According to Thornhill’s findings, striking at the root of infectious disease threats is by far the most effective form of social engineering available to any would-be reformer.
If you were looking for a paradigm-shifting theory about human behavior, step right up. “Once we started looking for evidence that pathogens shape culture,” Thornhill told me, “we began to find it in damn near every place we looked.”
“You still hear people say that the old South will rise again,” Thornhill says. “But I doubt it has a chance unless disease prevalence goes up dramatically.”
THORNHILL WAS STEERED TOWARD the topic of the human psychological reaction to disease in the early 2000s by a young graduate student advisee named Corey Fincher. Fincher had arrived at the University of New Mexico intending to study the mating behavior of rattlesnakes. After a time, however, he instead became curious about the evolutionary effects of disease on human cultural behavior—and particularly about the question of why cultures tend to fall along a spectrum between individualist and collectivist dispositions.
Psychologists and other social scientists have long been curious about this robust difference between human populations. In strongly collectivist societies, group membership forms the foundation of one’s identity. Sacrificing for the common good and maintaining harmonious ties with family and kin are expected. By contrast, in strongly individualist societies like those of the United Kingdom, the U.S., Australia, and the Netherlands, individual rights are valued above duties to others. One’s identity does not derive from the group, but rather is built through personal actions and achievements. Although these differences have been confirmed by many cross-cultural studies in a variety of different ways, no one had come up with a convincing evolutionary theory to suggest why it would be advantageous for one group of people to become more collectivist and another group to become more individualist.
Fincher suspected that many behaviors in collectivist cultures might be masks for behavioral immune responses. To take one key example, collectivist cultures tend to be both more xenophobic and more ethnocentric than individualist cultures. Keeping strangers away might be a valuable defense against foreign pathogens, Fincher thought. And a strong preference for in-group mating might help maintain a community’s hereditary immunities to local disease strains. To test his hypothesis, Fincher set out to see whether places with heavier disease loads also tended toward these sorts of collectivist values.
Working with Damian Murray and Mark Schaller, two psychologists from the University of British Columbia, and Thornhill, Fincher compared existing databases that rated cultural groups on the individualist-collectivist spectrum with data collected from the Global Infectious Diseases and Epidemiology Network and other sources. The team paid special attention to nine pathogens (including malaria, leprosy, dengue, typhus, and tuberculosis) that are detrimental to human reproductive fitness. What the team found was a strong correlation between collectivist values and places with high pathogen stress. In 2008, Fincher, Thornhill, Schaller, and Murray published a major paper in Proceedings of the Royal Society B that laid out the connection.
Thornhill and Fincher found further evidence for the pathogen stress theory by looking at geographical regions that had not only severe disease stress but also a highly diverse patchwork of local pathogen populations. The critters that make us ill—not only the viruses and bacteria, but also the ticks, flies, and mosquitoes that spread them—are tiny and lack the ability to regulate their own heat as larger organisms do. They often flourish only in very narrow climatic zones, where they are adapted to certain temperature and moisture levels. As a result, pathogen threats can be highly localized. One study, for instance, found at least 124 genetically distinct strains of the parasite Leishmania braziliensis across Peru and Bolivia.
If you were to live in such a pathogenically diverse place, you and your family would likely develop a resistance or immunity to your local parasites. But that defense might be useless if you were to move in with a group just a short distance away—or if a stranger, carrying a foreign pathogen load, were to insinuate himself into your clan. In such places, then, it would be important for neighboring groups to be able to tell the difference between “us” and “them.” With that thought in mind, Thornhill and his colleagues made a prediction: that regions with a balkanized landscape of localized parasites would in turn display a balkanized landscape of localized customs and conspicuous cultural differences among human populations—dialects, unique religious displays, distinctive art and music, and the like. While there is much more research to be done, early findings suggest that—particularly when it comes to the development of local languages and religions—pathogen stress does appear to spawn cultural diversity.
A set of more cautious researchers would likely have circled the wagons after unveiling their theory and concentrated on building a body of evidence to defend their early claims. Having a novel explanation for why some cultures are collectivist while others are individualist would probably guarantee one’s place in social science lore. Thornhill and Fincher, however, didn’t stop for a breath. By the time the two published a major paper in Behavioral and Brain Sciences in 2012, they had marshaled evidence that severe pathogen stress leads to high levels of civil and ethnic warfare, increased rates of homicide and child maltreatment, patriarchal family structures, and social restrictions regarding women’s sexual behavior. Moreover, these pathogen-avoidant collectivist tendencies, they wrote, coalesce over time into repressive and autocratic governmental systems. Want to understand the rise of fascism, dictatorship, and ethnocentric campaigns that dehumanize outsiders? Look to the prevalence of pathogen threats. Over the years, scholars like William H. McNeill and Jared Diamond have argued that germs and geography exert an under-appreciated influence on the rise and fall of societies. But for Thornhill and Fincher, human psychological adaptations to the threat of disease are nothing less than the missing link in our understanding of culture—a fundamental key to our collective values that researchers and philosophers over human history have overlooked.
Over the last few years, an increasing number of papers from other social scientists have backed the theory. While many of these researchers work with the same large data sets and long timescales that Fincher and Thornhill study, others have figured out ways to tease out the behavioral immune response in real time, on a smaller scale. Schaller and his colleagues, for example, set up a test to see if disease cues could influence laboratory subjects’ opinions of foreigners. Schaller’s team had one group of subjects watch a slideshow about germs and disease while another group watched a show about everyday accidents and dangers. The researchers then told the subjects that the Canadian government was going to spend money to attract immigrants to the country. As Schaller predicted, the test subjects who had been cued with the disease presentation were less inclined to spend money to attract people from unfamiliar countries.
Many researchers remain unconvinced that pathogen stress is as important at Thornhill and Fincher suggest. Voicing a common critique, the anthropologist Scott Atran has argued that Thornhill and Fincher’s make “too precipitous a leap from correlation to cause” in their analysis of data. Other critics have pointed to potential counterexamples to the pathogen stress theory: If strong religiosity is, as Thornhill and Fincher claim, an adaptive response to pathogen stress, then why do some religious people behave in such pathogenically promiscuous ways—engaging in blood rituals, circumcision, piercing or tattooing, or tromping off to proselytize in strange lands? Still other researchers have stepped in to suggest that the level of in-group preference in a culture can be better understood in relation to the quality and accessibility of local governmental institutions: The more dependable the institutions, the less people have to invest in their family and local groups to meet basic needs.
The most effective way to change political values from conservative to liberal, Thornhill says, is through health-care interventions.
During our interview at the zoo, Thornhill appeared neither boastful about his theory nor particularly defensive about criticism. At 69 years old, he is the picture of an avuncular, somewhat rumpled professor, happy to spin out his ideas. At this stage in his career, he said, he no longer spends time worrying that other social scientists are not yet on board or that they think he may be overreaching. He is fond of quoting Albert Einstein, who once said, “the grand aim of all sciences is to cover the greatest number of empirical facts by logical deduction from the smallest number of hypotheses or axioms.” Of course, grand hypotheses can be easy to come up with: just eavesdrop outside any dorm-room door. Theories that actually explain broad patterns in nature, Thornhill acknowledges, are extremely rare. But he is convinced he has one of those extraordinary beasts by the tail. Other social scientists, he tells me, will eventually catch up. (He and Fincher are at work on a magnum opus about their theory that they hope will be out within the year.)
Thornhill is no stranger to controversial theories. While he made his professional name in 1983 with a groundbreaking book called The Evolution of Insect Mating Systems, he became something of a public figure in 2000 by publishing a book, with the anthropologist Craig T. Palmer, entitled A Natural History of Rape: Biological Bases of Sexual Coercion. Their argument—that rape needed to be understood as an evolved sexual strategy in males—flew in the face of the politically correct view of the day: that rape could only be seen as an pathological act of violence and control. Talk-show bookers loved the authors’ controversial take, and Thornhill gamely ran the gauntlet, even appearing on The Today Show to explain the theory.
Popular reaction to the idea was heated. Evolutionary behavioral theorists often struggle against the misapprehension that describing a behavior as “evolved” is the same thing as justifying it. Thornhill was no exception. As much as he tried to be clear that he was not making an excuse for sexual assaults, he received a series of death threats on his answering machine, and one detractor apparently attempted to break into his home. The university assigned a campus police officer to walk him to and from class. Thornhill, who even admirers call an “academic cowboy,” was shaken but not cowed.
While the pathogen stress theory has not reached The Today Show level of pop-cultural consciousness—nor anywhere near the same level of controversy—it has certainly ruffled feathers within the social sciences. The scope of the theory is so broad, and its claims are so dramatic, that it treads on virtually everyone’s intellectual turf. “It really crosses disciplines,” Thornhill admitted. “Economics, political science, psychology, anthropology. Each of these fields has different assumptions about how culture works and how they’ve come to be like they are. For that reason, it is sometimes very hard to break into those inter-disciplinary debates. You just have to hope that the evidence will win out.”
SOME ACADEMICS HAVE SUGGESTED that while the pathogen stress theory might help explain some of the deep origins of human history, it has little relevance to the modern era. Indeed, in much of the developed world today, major pandemics and many historically common infectious diseases are largely a thing of the past. Chronic conditions like diabetes, cancer, obesity, and heart disease are by far the largest health concerns. Walk into an American hospital suffering from malaria or the measles and you will more than likely be regarded as an oddity; it’s quite possible that no one on the staff will have any personal experience treating your condition.
But in Thornhill and Fincher’s view, it’s not just the threat of infection that shapes culture. The absence of disease threats, they argue, creates a different set of cultural conditions that, taken together, are the necessary precursors to modernity. Collectivist values, despite their potential effectiveness at fencing out disease, come at a steep cost to the cultures that harbor them. As Thornhill explained to me, keeping strangers at arm’s length can limit trade and stymie a culture’s acquisition of useful new technologies, materials, and knowledge.
So, as humans moved into drier and colder and less disease-ridden climates, Thornhill says, they likely discarded their costly xenophobic disease-avoidant ways and became less beholden to tradition, more willing to trade with others, and more accepting of technological innovations. Instead of censuring the individual maverick thinker in the group, societies eventually came around to rewarding those who challenged convention. With those changes came the rise of wealth and the spread of education to a larger and larger segment of the population. The more educated the population, the more people demanded participation in their governments. Democracies, premised upon the rights and freedoms of individuals, were the natural outcome.
Moreover, the democratizing effect of lowering disease threats, they argue, can happen quite quickly—even within a generation. Freedom House, an organization that tracks governments, civil liberties, voter participation, and equality around the globe, considers 46 percent of all countries to be “free” today, as opposed to just 29 percent in 1972. Thornhill points out that this rise coincided with an era in which major health interventions, including vaccine programs, the chlorination of drinking water, and efforts to reduce food-borne disease, became commonplace in many parts of the world. Thornhill is not shy about the implications. If promoting democracy and other liberal values is on your agenda, he says, health care and disease abatement should be your main concern.
“Once we started looking for evidence that pathogens shape culture,” Thornhill told me, “we began to find it in damn near every place we looked.”
ONCE YOU BECOME AWARE of the pathogen stress theory, it has a kind of earwormish power. Even the most obvious counterexamples that spring to mind can, on closer inspection, seem to offer oblique and even surprisingly overt support for some version of the pathogen stress theory. It’s rather conspicuous that Nazi Germany—probably the most famous modern example of an ethnocentric, bellicose, authoritarian regime—arose in a northern clime, and not in some tropical latitude. But consider that the Nazi party began its rise to power in the aftermath of a Spanish flu pandemic that had killed over two million people across Europe—over half a million in Germany alone. And remember that much of Hitler’s poisonous rhetoric specifically suggested that Jews were disease carriers. Again and again, his rants portrayed Germany as an organism fighting disease—caused, among other things, by “Jewish bacteria.” Did Hitler manage to manipulate an unknown psychological mechanism that had been triggered by the threat of disease in the German population?
There are several disquieting aspects to Fincher and Thornhill’s theory. Fincher is careful to say up front that their hypothesis is not meant to telegraph value judgments or guidance, but it’s hard not see the pathogen stress theory’s distinction between collectivist and individualist societies as a kind of politically charged, world-historical morality play. On one hand you have collectivist cultures rife with xenophobia, racism, adherence to authority, and restrictive religions. On the other side are liberal cultures that promote equality, open-mindedness, democracy, and the acceptance of outsiders. One set of cultural values is a psychological defense against sickness; the other, a logical extension of life in a healthy society. In this light, the pathogen stress theory can seem to offer evolutionary justification for the cultural values that Thornhill and Fincher themselves espouse—a reminder, some might say, that not only history, but science as well, is written by the victors. But for his part, Thornhill is confident in the evidence underlying his theory, and relatively untroubled by the implications. “If you increase health then people will become more liberal and happier,” he told me at the zoo. “I don’t think that is a bad idea.”
The pathogen stress theory is also hard to swallow in a way that evolutionary psychology arguments often are—especially for those who fancy the idea that we are in control of our thoughts, emotions, and behaviors. The next time someone tells you about their religious beliefs, try convincing them their firmly held convictions spring from an unconscious disease-avoidance mechanism. Or, alternatively, try telling a liberal acquaintance that their beliefs about openness and inclusion are only as deep as the good luck that has allowed them to live in a relatively disease-free zone.
“It is true that the pathogen threat theory doesn’t integrate with the profundity we feel when we talk about values,” Thornhill admitted while eating a sandwich at the zoo cafe. “When we think about our religious or political beliefs we feel like we’ve decided on them. They don’t feel like a defense against disease. They feel like something more meaningful. They feel like the truth.”
Thornhill and Fincher’s analysis takes such a satellite view of humanity—deriving its insights from vast data sets—that it can feel alien and cold. Thornhill started his career as a behavioral ecologist studying insects, and his perspective on human populations can at times seem every bit as distant as his perspective on the anonymous populations of bugs he once investigated. There is, however, at least one respect in which Thornhill’s research comes down to a deeply personal, human scale.
Thornhill grew up in Alabama in the 1940s and ’50s. He says he witnessed firsthand the rank sexism, racism, and xenophobia that was rampant in the South during that period. And he is well acquainted with the region’s strong family ties and firm religious beliefs. But he is also aware of a somewhat lesser known set of facts about his native soil. Around the time of his childhood, Southern states were finally getting a hold on a pair of diseases that had long plagued the region: malaria and hookworm. These diseases, writes Peter Hotez, the founding dean of the National School of Tropical Medicine at Baylor University, had turned generations of Southerners into “anemic, weak, and unproductive children and adults.” Not surprisingly, Thornhill believes that the collectivism of the old South—the adherence to tradition, ethnocentrism, and suspicion of outsiders that marked his childhood—stemmed from its historically high pathogen load.
Similarly, he attributes the progress he’s seen toward a more egalitarian South to the alleviation of the region’s most pernicious health problems. “You still hear people say that the old South will rise again, but I doubt it has a chance unless disease prevalence goes up dramatically,” he says. “Maybe if you knock out all the sewage treatment plants and stop giving antibiotics to sick kids, it would have a chance.”
As fortune would have it, the United States may have just embarked on a natural experiment to test Thornhill and Fincher’s pathogen stress theory. Conservatives (with their collectivist values emphasizing religion, tradition, and regionalism) and liberals (with their individualist values of openness, anti-authoritarianism, and experimentation) have spent the better part of 10 years now manning their battle lines over the issue of universal access to health insurance coverage. If Thornhill and Fincher are right, conservatives may have had more reason to oppose the Affordable Care Act than they currently understand. Might an effective health intervention such as Obamacare move the country, on some deep psychological level, away from conservative values and toward more liberal ones? Is it possible that there are utterly unacknowledged stakes in this battle?
Thornhill has pondered these questions privately. While he’s not sure whether Obamacare is strong enough medicine to move the dial significantly in terms of disease levels, he is certain that the most effective way to change political values from conservative to liberal is through health-care interventions and advances in providing clean water and sanitation. “That is clearly the conclusion that the bulk of evidence supports,” Thornhill says. “If you lower disease threats in countries they become more liberal, and that is true for states in this country. The implication is that if you effectively target infectious diseases then you will liberalize the population.”
At the same time, well beyond the borders of the United States, the coming decades may supply a wholly different test of the pathogen stress theory. Higher temperatures, elevated sea levels, and increased precipitation in some areas—all predicted to accompany climate change—are expected to bring tropical diseases to higher latitudes and elevations in the coming decades. Pathogens that once perished in cold climates and dry soils may find new congenial zones of heat and moisture, and new host populations. Incidents of dengue fever in the U.S., for example, are expected to spread beyond Hawaii and the Mexican borderlands as climate change creates expanding habitats for the mosquito that carries the virus. Unless effective health interventions ward off these new threats, humans in ever higher latitudes may again have to resort to their embedded psychological and cultural defenses. Collectivist group behaviors may yet stage a comeback.
This post originally appeared in the March/April 2014 issue of Pacific Standard as “The Germ Theory of Democracy, Dictatorship, and Your Most Cherished Beliefs.” For more, subscribe to our print magazine.