Parasites in the Pew

Something Rotten in the State of Nature

by by Soma Chea

illustration by by Robert Sandler

Many scientists have been wary about using evolution to study human nature. But not Randy Thornhill. Author of The Evolutionary Biology of Human Female Sexuality and A Natural History of Rape: Biological Bases of Sexual Coercion, Thornhill is part of an expanding circle of researchers who believe that human nature can be explained by modern evolutionary theory. They call themselves the Human Behavior and Evolution Society. The members of this society are experts who come from across the globe and represent many different disciplines of study including biology, psychology, anthropology, medicine, economics, and sociology. Thornhill, who was accused by another scientist of “endorsing rape” in his book on sexual coercion, is no stranger to controversy, nor is he deterred in his quest for knowledge. Thornhill, along with Corey Fincher, another professor of Biology at the University of New Mexico, has recently published an article in the peer-reviewed journal Behavioral and Brain Sciences that reveals a peculiar link between parasites and religion. The paper suggests that stress on a population caused by parasitic infections promotes behavior that includes strengthening religious activity and commitment.

It seems hard to believe that mere worms or single-celled animals—so small that it takes a microscope to see them—can have such far-reaching impacts on human behavior. Until the last five years, scientists had no proof that parasites had been infecting people for most of human existence. But new evidence of lung fluke eggs found in fossilized feces put the earliest parasitic infections to at least 5,000 years before Christ. For scientists like Fincher and Thornhill, this meant 5,000 years of human-parasite co-evolution. Parasites and their human hosts have been continually locked in a biological arms race—each adapting and evolving through the ages to meet the challenges of the other. One of the mechanisms of protection against parasites, as Fincher and Thornhill have suggested, is the formation of social groups and religious structures.

The defenses against parasitic infections includes not only the classical immune system—the biochemical, tissue, and cellular responses to invading organisms—but also, an additional level of protection called the behavioral immune system. The idea of a behavioral immune system isn’t new. Mark Schaller, another evolutionary biologist who can be credited for coining the term, described it as behavioral and psychological disease avoidance. The phrase, applied to Fincher and Thornhill’s study, can be understood as the anti-parasite psychology and behavior that reduces the risk of infection in an individual. Included in these instincts are the negative attitudes and biases against organisms and objects in environments that from sensory cues are judged to be unclean or likely to be infected with parasites. The behavioral immune system explains why we get that stomach-churning feeling at the sight of blood, an open wound, or a flowing pustule. It connects the dots between unpleasantness and avoidance. Is human waste and rotten food inherently bad-smelling or are they unpleasant because they should be avoided?

The preventative measures of our adaptive psychologies might have more profound impacts on the decisions that determine social grouping. Another aspect of Fincher and Thornhill’s theory is the notion that the behavioral immune system supports favoritism toward individuals within a social group and avoidance and bias against individuals outside it.

The idea returns to the concept of host-parasite co-evolution—the constant back-and-forth interactions, evasions, and adaptive maneuvers humans and parasites both use to gain leverage over the other. Thinking about these social formations as products of in-grouping behavior, Fincher and Thornhill were able to establish correlations among the amount of parasite stress on a population, the relative numbers of different religions, and the proportion of religiously affiliated individuals in a given region. Both the U.S. and the cross-regional study found that the higher the parasite stress, the broader the diversity of religious groups and the higher the percentage of religious affiliations.

Participation in religious communities, Fincher and Thornhill explain, may provide and reinforce critical knowledge on a population that can help to prevent and contain local infections. Through the teaching and inheritance of customs, norms, and values, group formations like religion determine the rules of an abiding population. Such collective knowledge may help to combat infections. Methods of cooking and rules on sexual practices and sanitation are some examples of insider knowledge that may prevent an individual from becoming infected with parasites. Outsiders without this knowledge are inherently at a disadvantage for infections as their own customs and norms may not completely align with an unfamiliar environment.

Some of the best-known religious texts show that parasites have made profound impacts on religious societies. Dietary restrictions and suggestions appear throughout the religious canon—many of which have practical implications against parasites and infectious disease. The Talmudic laws against eating pork may have been a way to prevent the spread and infection of Trichinella siralis, a worm commonly transmitted by pigs that can cause painful inflammation of heart muscles and swelling of the brain. The Bible also mentions fasting and quarantine—mechanisms that would have been beneficial in controlling some parasites. In some Hindu cultures, nagas are worshipped for their powers, including guarding water supplies.

If we are to accept Fincher and Thornhill’s findings, how would they change the thinking behind our actions? Is there a biological explanation for xenophobia and ethnocentrism or a biological motivation for religious formation? Even if this were true, acknowledging an idea neither proves or disproves these implications. Such questions must be raised if any progress is to be made in understanding the human mind. Self-motivation, awareness, and the ability to produce complex thoughts are commonly identified as traits that set us apart from other animals. But it is due to these traits that such ideas exist.

SOMA CHEA B’15 has been infecting people for most of human existence.