In "This Is Your Brain on Parasites: How Tiny Creatures Manipulates Our Behavior and Shape Society," Kathleen McAuliffe presents a riveting investigation of the myriad ways that parasites control how other creatures—including humans—think, feel, and act. The book is both a journey into cutting-edge science and a revelatory examination of what it means to be human. Below is an excerpt from McAuliffe's "This Is Your Brain on Parasites" (Houghton Mifflin Harcourt, 2016).

Parasites held no interest for Mark Schaller at the outset of his career. Since his graduate-school days in the 1980s, the University of British Columbia psychologist has wanted to understand the root causes of prejudice. In one study that he conducted in the early 2000s, he showed that simply turning off the lights in a room made people more prejudiced against other races. Subjects' heightened sense of vulnerability in the dark seemed to elicit these negative biases — "a relatively obvious idea," he admitted. Then an odd thought occurred to him: "People are potentially vulnerable to infection. Wouldn’t it be cool and novel if we found out that prejudices are jacked up when people are more vulnerable to disease?"

As these ideas were taking shape in his mind, trailblazing experiments by psychologist Paul Rozin at the University of Pennsylvania and other scientists were increasingly suggesting that the raw, visceral emotion we call disgust evolved to protect us from pathogens and other parasites. According to this view, our minds are constantly on the lookout for contaminants—for example, rancid odors, an unflushed toilet, or garbage spilling out of a trash can—and when it detects them triggers revulsion and withdrawal from the threat. Maybe, the Canadian researcher thought, he could disgust subjects and then see if their attitudes toward outgroups— those perceived to be racially or ethnically different from themselves — shifted in a negative direction.

To that end, he showed participants a slideshow of snotty noses, faces covered in measles spots, and other disease-related stimuli. The control group saw pictures depicting threats unrelated to infection — for example, electrocution or being run over by a car. All the subjects were then asked to fill out a questionnaire that assessed their support for allocating government funds to help immigrants from Taiwan and Poland (groups whom they ranked as very familiar, as Vancouver is also home to many transplants from Eastern Europe) versus immigrants from Mongolia and Peru (whom they rated as unfamiliar). In comparison to the controls, the subjects who saw the germ-evoking photos showed a sharply elevated preference for familiar immigrant groups over lesser-known ones.

Drawing on more than a decade of research by himself and others since the study was published, Schaller offered this interpretation of the findings: Over human history, exotic people have brought with them exotic germs, which tend to be especially virulent to local populations, so foreignness seems to trigger prejudice when we feel at greater risk of getting sick. Also, it may be that lurking in the backs of our minds are concerns that the foreigner does not have as high standards of hygiene or that he doesn’t follow culinary practices that reduce the risk of food-borne illness. Prejudice, Schaller points out, is all about shunning others based on superficial impressions, so the sentiment, ugly as it is, is ideally suited for the purpose of shielding us from disease.

Related trials suggested that the mind’s sense of "foreign" is blurry. Schaller, in collaboration with other researchers, discovered that any reminder of our susceptibility to infection makes us more prejudiced against the disabled, the disfigured, the deformed, and even the obese and elderly — in short, a vast swath of the population who pose no health threat to anyone.

"Infectious disease causes a wide variety of symptoms so we're probably picking up on the fact that the person is not looking normal," he said. By normal, he means a caveman’s notion of what a healthy person should look like. Until very recently, "the prototype human being" — as he puts it — was rarely overweight or much older than forty, so people who are obese or show signs of old age, like bags under the eyes, liver spots, and curled yellow nails, are categorized as weird. Like a smoke detector, your germ-detection system is designed to sound at the least hint of danger. A false alarm could mean a lost social opportunity, but if someone displays contagious symptoms that you mistakenly think are innocuous, it could cost you your life. "Better safe than sorry" seems to be nature's motto.

To Schaller, it's "mind-boggling" that scientists have only recentlycome to appreciate that parasites in our surroundings might inflame prejudice, given that they've known about other behavioral defenses against disease — especially in animals — for decades. Viewed from a different angle, however, the oversight didn’t surprise him. "A lot of what people study is based on their own personal experience, and most work in the psychological sciences is done in Canada, the U.S., and Europe in places like this," he said, casting his eyes about. We were seated in a sparkling new building on the UBC campus with austere modern lines and sleek, minimalist decor — about as sterile a setting as one could imagine. "We don't really worry much about infectious disease. We forget that in most of the world and throughout most of our history, infectious organisms have posed this extraordinary health threat and have almost certainly played a huge role in human evolution, including the evolution of our brain and nervous system." Schaller coined the term the behavioral immune system to describe thoughts and feelings that automatically spring to mind when we perceive ourselves to be at risk of infection, propelling us to act in ways that will limit our exposure.

While he clearly thinks insights from this domain have much to teach us about interpersonal relations, he’s careful not to oversell his findings. A subconscious fear of contagion, he underscores, is hardly the sole cause of prejudice. We may negatively stereotype different races or ethnicities out of anger that they may threaten our livelihoods or out of fear that they may want to harm us. We may shun the disfigured and deformed because they are reminders of our own vulnerability to injury and misfortune. Or prejudice may simply be born of ignorance — the denigration of the obese as lazy and slovenly, for example, may stem from someone having little contact with overweight people in a professional setting. Even if we could banish the world of infectious disease, said Schaller, it wouldn’t eradicate prejudice.

He offered an additional caveat: "A lot of the research we've done has focused just on our initial automatic response to people who activate our behavioral immune system, but that doesn't mean that’s all that is going on in our heads. For example, my initial response to someone who is weird-looking might be revulsion, but that may be immediately superseded by a more deep sympathetic response that takes into account the predicament the person is in and can elicit sensitivity and understanding. These additional, more thoughtful responses may not be the first things that cross our psychological radar but they may ultimately have a much bigger effect on how we respond in real life in that situation."

Nonetheless, studies by Schaller and other researchers indicate that people who chronically worry about disease are especially prone to antipathy toward those whose appearances diverge from the "normal" template, and these people have a harder time moving beyond that reaction. This can have real, long-lasting effects on their attitudes and experiences. Compared to people not plagued by such health concerns, they are less likely to have friends who are disabled; by their own accounts, they are less inclined to travel abroad or engage in other activities that might bring them into contact with foreigners or exotic cuisines, they more frequently display negative feelings toward the elderly on tests of implicit attitudes, and they report harboring greater hostility toward the obese. Indeed, the more they fret about getting sick, the greater their expressed disdain for the obese, possibly explaining why fat people are so frequently branded with pejorative adjectives strongly linked to infection, such as dirty, smelly, and disgusting. These antipathies affect how germaphobes interact with everyone, not just strangers. Parents prone to such fears report having more negative attitudes toward their fat children — sentiments that don't carry over to their normal-weight offspring.

The recently ill display similar biases, possibly, Schaller theorizes, because their immune systems may still be run-down, so their minds compensate by ratcheting up behavioral defenses. In support of that contention, he points to a provocative study by evolutionary biologist Daniel Fessler and colleagues, who showed that pregnant women become more xenophobic in the first trimester, when their immune systems are suppressed to prevent rejection of the fetus, but not in later stages of gestation, when that danger has passed. Further research by Fessler in collaboration with Diana Fleischman revealed that the hormone progesterone, which is responsible for reining in the immune system early in a pregnancy, elevates feelings of disgust, which in turn promotes negative attitudes toward foreigners and pickier eating habits — the latter response likely an adaptation that discourages pregnant women from consuming foods prone to contamination, as we saw in chapter 8. In other words, it appears that by evoking disgust, a single hormone initiates two behavioral defenses at exactly the time in pregnancy when the danger posed by infection is greatest.

Such hormone-induced shifts in feelings are not limited to gestation. During the luteal phase of a woman's menstrual cycle (the days that follow the release of an egg from her ovaries), progesterone rises to allow an egg, should it become fertilized, to implant in the womb without being attacked by immune cells. By measuring salivary levels of the hormone in regularly cycling women, Fessler and Fleischman discovered that the luteal phase is accompanied by heightened feelings of disgust, xenophobia, and concern about germs. For example, women at that stage in their cycles reported more frequent handwashing and use of paper seat liners for toilets in public restrooms. "Understanding the sources of some of these attitudinal changes is potentially important," said Fessler. "In teaching my undergraduate students about how to understand the mind from an evolutionary perspective, I try to make the point that we’re not slaves to our evolved psychology. When a woman walks into a ballot booth to make a decision about a candidate based on his or her immigration policies, for example, this knowledge gives her the power to step back and say, 'Well, wait a minute. Let me make sure my decision reflects my well thought out position on this issue, and not impulses I’m experiencing at this moment.'"

Political scientists are now streaming into the field, testing whether central findings hold up across different cultures and in populations much larger than typically studied by psychologists. One of the biggest and best-controlled of these trials, conducted by Michael Bang Petersen and Lene Aarøe at Aarhus University, included nationally representative samples of two thousand Danes and thirteen hundred Americans whose vulnerability to infection was assessed by multiple measures. Next they completed a test designed to reveal xenophobic tendencies. The scientists' findings dovetailed neatly with Schaller's results from laboratory studies. In both samples, opposition to immigration increased in direct proportion to disgust sensitivity.

Although research on disgust has grown immensely in recent years, important questions remain unanswered — chief among them, does this stomach-churning emotion influence the functioning of our immune cells? In other words, is the psychological immune system communicating with the physical immune system, or do they operate largely independently of each other?

Unfortunately, researching these issues is difficult to do; it's costly and requires expertise beyond the scope of many psychologists. By enlisting the assistance of a team of neuroimmunologists, however, Schaller succeeded in conducting one of the few studies to address the question. As in many of his previous trials, subjects were shown a disease-y slide show, but with one major difference: Immediately before and after the presentation, their blood was drawn and mixed in a test tube with a pathogen surface marker to determine how aggressively their white blood cells countered the challenger. Specifically, the investigators looked to see if arousing subjects’ disgust spurred their white blood cells to produce higher amounts of a pathogen-fighting substance called interleukin 6 (IL-6).

It did — and by a whopping 24 percent. By comparison, the control group of subjects, who saw pictures of people brandishing guns pointed directly at them, showed virtually no change in IL-6. Interestingly, said Schaller, although the germ-evoking pictures were much more effective in revving up the immune system, the gun photos were actually judged to be more distressing, demonstrating the specificity of the immune response.

If, as this research implies, the disgusted mind truly can shift the immune system into high gear, it makes very good sense, in Schaller's opinion. "Our eyeballs are providing useful information to our immune system. If they're telling us there are a lot of sick people or other sources of germs around, that indicates that we ourselves are likely to become exposed or maybe already have been, so ramping up the immune system gives it a head start in fighting off microbial invaders." He thinks there may be another virtue to this biological setup as well. "The information allows the immune system to calibrate the aggressiveness of your response to the scale of the threat. We don't want the immune system working hard unnecessarily because it consumes a lot of resources that might be used by other parts of the body."

How, at a neurological level, the psychological immune system might "talk" to the physical immune system is still a matter of speculation. But scientists have begun to track where disgust is processed in the brain, and evidence suggests this region may also serve the function of making us repulsed by cruel or unethical people like corrupt politicians, shoplifters and wife beaters. Strange as it sounds, disgust may have played a major role in transforming our species into the most freakish of creatures: a moral animal.

Copyright © 2016 by Kathleen McAuliffe. Used by permission of Houghton Mifflin Harcourt Publishing Company. All rights reserved.