The Centers for Disease Control on Tuesday announced 20 new cases of measles, bringing the total to 141. It's the largest outbreak of measles in the U.S. since 2000, owing largely to the nation's growing anti-vaccination movement. These parents are afraid that vaccinations put their children at risk of developing autism, but new research that should relieve some of those fears.

Non-vaccinating parents are a small group. Pew Research reports that only 9 percent of Americans believe that vaccines are unsafe, and the Centers for Disease Control reports that all but 8 percent of young children have been vaccinated for measles. However, vaccination rates have declined slightly in a few advanced countries, including Canada. 

If you read the scholarship on the anti-vaccination movement, if you talk to anti-vaccinators, or if you read their websites, you’ll find that many of these parents believe that some childhood diseases whose origins remain uncertain—like autism—are environmental diseases (as opposed to genetic or infectious diseases). Vaccination, they believe, exposes children to toxic agents that cause autism. 

For evidence, they point to data from the CDC showing that the rate of autism diagnoses more than doubled from 2000 to 2010:

The number of immunizations children get have also increased dramatically: Whereas there were only five recommended vaccines in 1960, today a child may receive 24 shots by age two. Anti-vaccinators question whether the “sudden onslaught” of immunizations, as they describe it, is causing the epidemic growth of autism. 

It’s not impossible. But historical correlations provide little evidence of what causes a disorder. Consider that from 2000 to 2010, children have also been exposed to increasing levels of fluorinated gas emissions. Could that instead explain the increase in autism diagnoses? 

The key word there: "diagnoses." We don't know whether autism itself is increasing, or whether we're just getting better at diagnosing it. In a recent study published in JAMA Pediatrics, Aarhus University statistician Stefan Hansen and his colleagues examined the health records of more than 670,000 Danish children born from 1980 to 1991 to see whether the increase in autism rates could be explained by changes in how autism is diagnosed. Denmark was an ideal place to carry out this research because Scandinavian countries keep comprehensive lifetime medical records for all children based on uniform diagnostic procedures. 

The investigators found that just as in the U.S., there was an increase in the rate of diagnosed autism among Danish children. However, 60 percent of the total increase in autism could be explained by a change in the diagnostic criteria for autism in 1994 and by a decision to include diagnoses made in outpatient settings in 1995. That is, at least 60 percent of the increase was attributable to specific one-time changes in how autism was identified and reported.

This doesn’t mean that 40 percent of the increase in autism was caused by increases in environmental toxins. We can't rule it out, but no environmental toxin has been definitively identified as a cause of autism. Much of the remaining 40 percent increase in the Danish study may well have resulted from better training of doctors about autism or increased awareness of autism among parents. 

What this does mean is that parents shouldn’t panic about the apparently explosive growth in the rate of autism. The best explanation for the increase is that we are getting better at finding autism. There is no reason to believe that increasing numbers of vaccinations received by children is causing autism to increase.

Overall, the research evidence against the idea that the measles, mumps, and rubella (MMR) vaccine causes autism is very strong. For example, a study in the New England Journal of Medicine looked at all children born in Denmark from 1991 through 1998 (more than half a million kids). They found no association between vaccination and autism. (Early data suggesting an association between the MMR vaccine and autism proved to be fraudulent.) 

So how can we convince anxious parents that it is safe to vaccinate their children? Of course we need to give them accurate information about vaccinations. But in a discouraging study, Dartmouth College Professor Brendan Nyhan found that providing corrective scientific information about vaccines increased parents’ misperceptions and, among anti-vaccinators, reduced their intention to vaccinate their children.

How can this be? Yale Law School Professor Dan Kahan has found that people find reasons to discount scientific evidence about policy issues that are deeply tied to their cultural identity. For instance, presenting evidence about global warming to groups of partisans increases the polarization of their views because we value our affiliations with our friends more than we value action on global warming. If I express a deviant opinion about global warming, I’ll experience the disapproval of my like-minded friends, whereas if I do nothing about global warming—well, what I do as an individual will have no detectable effect on the planet.

The moral, Kahan argues, is that

information must be transmitted in a form that makes individuals’ acceptance of it compatible with their core cultural commitments. It is not enough that the information be true; it must be framed in a manner that bears an acceptable social meaning.

For this reason, the contemptuous tone of much recent criticism of anti-vaccinator parents has, in my view, been counterproductive.

Parents' moral energy would be better spent on protecting children from ubiquitous toxins like lead, whose harmful effects on cognitive and behavioral development are well documented. But we shouldn’t be telling anti-vaccinators that there is something wrong with their intelligence or character. We should speak to them in ways that respect their commitments to their children’s health and their concerns about possible associations between environmental toxins and neurodevelopmental disorders like autism. We should explain that while concerns about vaccines are misplaced, parents have a right to call for further research on the origins of autism.