Public health officials may not be hitting on the right morals in their quest to get all children vaccinated.

Health researchers have borrowed from a psychological theory that helps explain the rise of Donald Trump to figure out what holds parents back from vaccinating their kids.

Since the 1990s, the number of parents seeking vaccine exemptions for their children has been climbing, pitting public health against influential anti-vaxxers in a proxy messaging war.

But it seems the doctors trying to persuade the vaccine-hesitant — still a tiny minority of parents — have mostly been failing: Either their messages are off or the anti-vaccine campaigns, circulating among sources like conspiracy theorist Alex Jones, are too powerful to counter.

The latest evidence of that failure has come in a large measles outbreak in Washington state. As of Friday, there were 41 cases in Washington’s Clark County, which borders Portland, Oregon. Most all of the cases involve children between 1 and 10 years old who had not been vaccinated.

Now researchers are borrowing a theory from social psychology (one that also coincidentally helps explain the rise of Donald Trump) to understand where doctors’ pro-vaccine campaigns may be going wrong. And they’ve discovered that they’re probably emphasizing the wrong things in their messages, as they describe in a study published in the journal Nature Human Behavior.

The paper, led by researchers at Emory University in Atlanta, was designed to tease out which morals parents who were hesitant about vaccines held most dear when making personal decisions, including about vaccinating their kids. The researchers turned to Moral Foundations Theory, an effort by social and cultural psychologists to chart the moral values that are common across cultures in guiding decision-making.

Moral Foundations Theory was widely cited in the aftermath of the election, when the facts alone had trouble explaining the rise of Trump. The theory involves six moral foundations, or “moral taste receptors”: care/harm, authority/subversion, loyalty/betrayal, liberty/oppression, purity/degradation, and fairness/cheating. For different people, different morals resonate more effectively. Conservatives are more likely to respond to appeals to authority and loyalty, for example, while liberals prize fairness.

The Emory authors behind the paper used a standard Moral Foundations Theory questionnaire to survey more than 1,000 parents of children under age 13 who were living in the US, asking them questions about whether something was right or wrong to suss out their values and then determine how important those values were in decision-making. They also asked the parents about their vaccine beliefs. An independent group of researchers, based at Loyola University in Chicago, repeated the survey on another group of American parents.

Together, they found the morals of purity and liberty were most associated with vaccine hesitancy, shedding light on what people who refuse vaccines care about — and what may be holding them back.

Parents who are concerned about vaccines prize liberty and purity

People who value liberty are concerned with individual freedom and resent others’ dominance over them, while those who value purity disapprove “of acts that are deemed ‘disgusting’ or ‘unnatural,’” the researchers wrote. (That squares with the findings of sociologist Jennifer Reich, who studies the anti-vaccine movement. She’s found vaccine-hesitant parents tend to favor natural and organic food, for example.)

“This is important because many of our go-to arguments about herd immunity and keeping your children safe are themed around values of harm and fairness,” said the Emory study’s lead author, epidemiologist Avnika Amin.

Public health campaigns typically seek primarily to educate on potential harms, reminding parents that getting immunized helps prevent outbreaks, and protects those who can’t be vaccinated, like people with allergies to vaccines or very young children.

But those may not be the values that resonate with vaccine-hesitant parents. The researchers found that parents who were concerned about vaccinating their children scored similarly to parents who weren’t when it came to the morals of harm and fairness.

Anti-vaccine messaging has emphasized purity and liberty, while public health campaigners have been focused on harm and fairness

On the flip side, the paper also helps explain why anti-vaxxers’ messages may appeal to this group of parents. As the researchers wrote, “Anti-vaccination websites also often claim that vaccines contain ‘contaminants’. These concerns may be rooted in the purity moral foundation, with its emphasis on avoiding anything disgusting or unnatural. Another frequent message on anti-vaccine websites is that mandatory vaccination policies violate parental civil liberties.”

The results suggest that pro-vaccine communications could be strengthened by taking a page from the anti-vax book — and presenting messages that better align with purity and liberty.

“You could increase the salience of disgust associated with certain diseases, and say vaccines fight those,” said the Emory study’s senior author, Dr. Saad Omer. “Or you could frame purity positively — saying vaccines are a very natural product, they work with a natural system. Messages that talk about liberty, that the freedom to choose for your child is being taken away if other others don’t vaccinate, might work.”

This finding is extremely important, considering that over the past three decades, the number of parents who opt out of vaccines for their kids has grown. In one 2009 New England Journal of Medicine paper, researchers looked at the state-level rates of nonmedical exemptions and found that between 1991 and 2004, those rates increased from less than 0.98 percent to about 1.5 percent. According to a recent study, we’re now hovering above the 2 percent exemption rate, which translates to thousands more unvaccinated children than just a decade ago.

As I’ve explained, vaccine refusers are a politically diverse group, but they do have some things in common. Parents who reject some or all vaccines are more likely to be white, college-educated, and married, with higher family income.

Both Omer and Amin emphasized that the moral messages they’ve identified will need to be tested, and that’s what they’re hoping to work on next. So keep an eye out to see if they’ve truly found the elusive key to effective vaccine communications.