Bill Gates drinking recycled water.

Early last month, Bill Gates released a video of one of the latest ventures funded by the Bill and Melinda Gates Foundation: the Omniprocessor, a Seattle-based processing plant that burns sewage to make clean drinking water. In the video, Gates raises a glass of water to his lips. Just five minutes ago, the caption explains, that water was human waste. Gates takes a sip. “It’s water,” he says. “Having studied the engineering behind it,” he writes, on the foundation’s blog, “I would happily drink it every day. It’s that safe.”

According to the Gates Foundation’s estimates, at least two billion people lack access to proper sanitation; a 2012 intelligence community report to the State Department warned that, within the next decade, “many countries important to the United States will experience water problems—shortages, poor water quality, or floods—that will risk instability and state failure.” The Omniprocessor’s approach seems to be the perfect solution. It offers proper waste disposal in place of contamination, and clean drinking water where access is lacking. In fact, the technology has been around for years, and its efficacy is an established fact. So why hasn’t it been widely adopted yet?

That’s precisely the question that Paul Rozin, along with Brent Haddad, Carol Nemeroff, and Paul Slovic, tackled in a series of studies spanning more than two thousand American adults and several hundred college students. The results were published, in January, in the journal Judgment and Decision Making. “The problem isn’t making the recycled water but getting people to drink it,” Rozin told me recently. “And it’s a problem that isn’t going to be solved by engineers. It will be solved by psychologists.”

In the first series of studies, the group asked adults in five cities about their backgrounds, their political and personal views, and, most important, their view on the concept of “recycled water.” On average, everyone was uncomfortable with the idea—even when they were told that treated, recycled water is actually safer to drink than unfiltered tap water. That discomfort, Rozin found, was all about disgust. Twenty-six per cent of participants were so disgusted by the idea of toilet-to-tap that they even agreed with the statement, “It is impossible for recycled water to be treated to a high enough quality that I would want to use it.” They didn’t care what the safety data said. Their guts told them that the water would never be drinkable. It’s a phenomenon known as contagion, or, as Rozin describes it, “once in contact, always in contact.” By touching something we find disgusting, a previously neutral or even well-liked item can acquire—permanently—its properties of grossness.

Feelings of disgust are often immune to rationality. And with good reason: evolutionarily, disgust is an incredibly adaptive, life-saving reaction. We find certain things instinctively gross because they really can harm us. Human secretions pass on disease. Noxious odors signal that your surroundings may be unsafe. If something feels slimy and sludgy, it’s likely a moisture-rich environment where pathogens may proliferate. Disgust is powerful, in short, because it often signals something important.

It’s easy, though, to be disgusted by things that aren’t actually dangerous. In a prior study, Rozin found that people were unwilling to drink a favorite beverage into which a “fully sterilized” cockroach had been dipped. Intellectually, they knew that the drink was safe, but they couldn’t get over the hump of disgust. In another experiment, students wouldn’t eat chocolate that had been molded to look like poop: they knew that it was safe—tasty, even—but its appearance was too much to handle. Their response makes no logical sense. When it comes to recycled water, for instance, Rozin points out that, on some level, all water comes from sewage: “Rain is water that used to be in someone’s toilet, and nobody seems to mind.” The problem, he says, has to do with making the hidden visible. “If it’s obvious—take shit water, put it through a filter—then people are upset.”

Disgust has deep psychological roots, emerging early in a child’s development. Infants and young toddlers don’t feel grossed out by anything—diapers, Rozin observes, are there in part to stop a baby “from eating her shit.” In the young mind, curiosity and exploration often overpower any competing instincts. But, at around four years old, there seems to be a profound shift. Suddenly, children won’t touch things that they find appalling. Some substances, especially human excretions of any sort, are seen as gross and untouchable all over the world; others are culturally determined. But, whether universal or culturally-specific, the disgust reactions that we acquire as children stay with us throughout our lives. If anything, they grow stronger—and more consequential—with age.

Toilet-to-tap water isn’t the only public-health advance being held back by human squeamishness. In places where protein is short, for example, insects can provide a needed source of nourishment; they are already abundant, and raising them commercially is environmentally friendlier than raising livestock. (Dana Goodyear explored the possibility of using insects as “mini-livestock” in a recent article for this magazine.) But most of us learn early on that insects are gross. In a study currently under review, Rozin, Matthew Ruby, and Christen Chan looked at people’s willingness to eat bugs in the United States and India. Less than half of the participants said that they would be willing to eat insects regularly for as little as a week—even if they were just mixed, in trace amounts, into flour. The negative attitude seems to be on the rise even in cultures in which eating insects used to feel natural; rates of insect consumption are falling. “As groups get more globalized and westernized, they start eating foods like us,” Rozin says. “They know insects are offensive to people they want to be like, and so they stop eating them.”

G.M.O.s, or genetically modified foods, are a third area where visceral disgust trumps all evidence and reason. In 2005, Rozin published a survey showing that, when it comes to “naturalness,” content is far less important than process; a natural substance can easily be rendered “unnatural” by passing through an unnatural-seeming transformation, even one as innocuous as boiling or pasteurization. In a forthcoming paper with Sydney Scott and Yoel Inbar, Rozin argues that the tendency to conflate naturalness with goodness is one basic reason G.M.O.s are facing such an uphill battle. Some people don’t like the corporations responsible for the spread of G.M.O.s. But for many others, according to his data, it’s simply a question of disgust: G.M.O.s go through an “unnatural” process—a transformation that changes them from one thing to another—and that very process makes them unpalatable, regardless of actual danger or evidence. More than seventy per cent of those who expressed opposition to genetically modified food—close to half of all of those surveyed—said that their view would not change regardless of the evidence put before them. G.M.O.s are unnatural and, therefore, disgusting.

“ ‘Natural’ is for some people like God, a fundamentally good thing. You shouldn’t play around with it,” Rozin says. “But consider this. The same people who claim that G.M.O.s are unnatural have cocker spaniels and eat corn, which is about as unnatural as anything you can find. How do you parse that in your mind? If natural means, for some people, not original state, we don’t eat anything at all that’s natural, other than truly wild plants.”