Parasites may have shaped religion’s evolution

Connor Wood


Religion, many commentators have complained, is frustratingly multifaceted. It can give profound meaning to life but, at the same time, pit rival human groups against each other. It can inspire hope for a better life and hatred for outsiders simultaneously. So why the seeming dark and light side of religion? Specifically, why do religions create sharp, often hostile divisions between groups? One research team from the University of New Mexico has found evidence that may surprise you. Hint: it has to do with creepy-crawlies.

Read MoreIf you’re reading this, you’re probably pretty lucky. Chances are you grew up in a neighborhood where most kids survived to adulthood, few if any illnesses took away your friends, and infections were easily treated with antibiotics or, for small cuts, hydrogen peroxide. But let’s face it – most of humanity hasn’t been so fortunate. The threat of death from illness and infection has been omnipresent for most human groups since we arose as a species; in fact, in many societies it hasn’t been unusual for only half of the children born in any generation to survive to adulthood.

Because so many of the deaths in human history were caused by infectious illnesses – such as the Black Death, tuberculosis, and cholera, to name just a few – many evolutionary scientists suspect that humans have evolved specific cultural ways of protecting their families and communities against the threat of pathogens. These scientists distinguish between what they call the “classical” and the “behavioral” immune systems. The classical immune system is what we learn about in biology and health class – it’s mostly comprised of white blood cells that attack foreign invaders. You could think of it as our internal immune system. The behavioral immune system, on the other hand, is the set of behaviors, emotions, and cultural norms that prevent us from coming into contact with nasty germs in the first place. This “external” immune system includes norms and taboos concerning food, dead bodies, and outsiders – anything that might introduce new pathogens into the community.

The behavioral immune system, then, may partially explain the strict divisions that often exist between different human societies. The argument goes like this: germs and other parasites tend to evolve alongside human groups, and so people often are born with immunities to the bacterial and viral infectious agents that live in their immediate environment. But they don’t necessarily have resistances to the germs from other areas, because they’re not exposed to them from birth. This means that people from other regions, even nearby ones, may be carriers of new and dangerous illnesses. Thus, one of the most important functions of the behavioral immune system may actually be to make people avoid members of other groups – in other words, to close themselves off to outsiders.

Corey L. Fincher and Randy Thornhill, both biologists at the University of New Mexico, describe three specific attitudes or behaviors that are associated with this group-protective aspect of the behavioral immune system. The first is philopatry, or the tendency to stay close to where one was born. Obviously, never leaving one’s home village or territory ought to significantly limit one’s exposure to strange bugs and viruses. The second is ethnocentrism, or in-group favoritism – the preference for socializing and living among people of your own kind. And the third is xenophobia, or the active fear and dislike of outsiders.

But plenty of people travel and hang out with people from other countries or ethnic groups, you might be thinking. And you’re right – these behavioral immune traits aren’t uniform across societies and locales. Fincher and Thornhill claim that levels of philopatry, ethnocentrism, and xenophobia vary across cultures because different regions have different levels of pathogens. In regions of the world where there are lots of different germs and parasites, societies tend to be smaller and more closed-off to outsiders, while in less germ-intensive places group boundaries are generally more porous.

Of course, nothing differentiates societies from one another more effectively than their religions. Religions often require people to wear specific clothes, adopt certain behaviors, and even modify their bodies in prescribed ways (think circumcision or ritual scarring). Many anthropologists think that these outward signs of religious commitment help to solidify religious in-groups, making them more resilient in the face of threats and more likely to overcome adversity. But these religious clothes, behaviors, and bodily modifications also help distinguish outsiders – thus serving as tribal or group identifiers.

Fincher and Thornhill recently published a research article in the journal Behavioral and Brain Sciences that combined a number of the above theories and findings to see whether religious practice and involvement would be higher in parts of the world where pathogens, parasites, and germs abound. They used data from the GLOBE project, the World Health Organization, the American Religious Identification Survey, and other preexisting databases and surveys to cross-reference religious adherence, religious participation, and the strength of family bonds against the prevalence of infectious diseases in different regions of the United States and the world.

They found that, even controlling for external factors such as economic output and income inequality, countries with higher levels of different infectious diseases had more religious and more family-oriented citizens than countries with fewer diseases. Within the United States, a similar dynamic was discovered: states with high levels of transmissible illness reported greater religiosity and more preference for socializing with family instead of with outsiders than states with relatively less parasitic danger.

An interesting correlate of these findings is the fact that parasitic threat is strongly associated with middle latitudes – that is, there are far more germs and parasites, and far more different varieties, in each square mile of the tropics than in each comparable square mile of more temperate latitudes. And this relationship is generally linear: the southern United States have a greater diversity and density of transmissible illnesses than, say, cold New England or Minnesota. And it’s no secret that the South is generally far more religious than the North or Upper Midwest.

Religion, then, with its ingroup/outgroup boundaries, may be one means of preventing people from associating too much with outsiders, since outsiders may carry potentially lethal pathogens. That regions of the world with more different pathogens per square mile tend to be more religious – and oriented to family and in-groups – suggests that this model may be onto something. It also may help make sense of the longstanding association between secularism and northern, cosmopolitan areas (specifically Western Europe and coastal North America): people may be less religious in these regions of the world partially because their climates and ecosystems just don’t pose as much threat to people’s immune systems. Thus, people don’t have as great a need to retreat into protective religious in-groups as they do in wetter, more biologically diverse (and hazardous) areas of the globe.

Still, despite the compelling implications of Fincher and Thornhill’s data, it’s important to remember that there’s a great deal more to religion than in-group identification. Their article doesn’t discuss the meaning-making functions of religion, nor the various ways religions help people to cope psychologically with existential threats like death and loss. And don’t forget the longstanding association of religion with art, music, and creative cultural expression – see the Sistine Chapel ceiling or the sheer exuberance of a Hindu Ramayana play. These more aesthetic or value-rich aspects of religion may not have much role to play in Fincher and Thornhill’s model, but they certainly count as part of the data of religion.

Assuming their conclusions hold in future testing, though, the relationship Fincher and Thornhill have found between the threat of transmissible illness and religiosity is a fascinating – and informative – one. Religion may be more than just a means of protecting its adherents from exposure to germs, but that may very well be one of its many functions. As usual, it’s not a question of “nothing but.” It’s a matter of “this and more.”


"This explanation jibes with my own mystical experiences brought on by meditation. Thanks for sharing."

How the brain’s predictions shape religion
"For me the mainline churches have become watered down psychological pablum. The preachers have painted ..."

Why IS liberal Protestantism dying, anyway?
"Everyone was already aware of how the brain works. That's how you can tell biological ..."

How the brain’s predictions shape religion
"Thanks Connor, fascinating account and very readable overview of the van Elk and Aleman paper ..."

How the brain’s predictions shape religion

Browse Our Archives

Follow Us!

What Are Your Thoughts?leave a comment