The recent outbreak of measles in Disneyland and the increase in measles cases nationwide have led to public health concerns, as well as a backlash against parents who refuse to vaccinate their children. Most media coverage has framed the issue as science versus irrationality, ignorance, and fear. However, this framing misses a key motivation for refusing vaccination. In some cases, the problem arguably isn’t the reluctance of anti-vaccine parents to accept overwhelming scientific evidence that vaccines are safe and effective. It’s that anti-vaccine parents are exploiting others by behaving rationally and strategically.

Public health experts see parents who refuse to vaccinate their children as irrational people who cannot accept clear and convincing evidence that vaccines are safe and effective. Public health experts encourage vaccination by emphasizing our vast historical experience with vaccines and the absence of significant side effects. Some commentators have linked parental decisions to refuse vaccines to broader skepticism about science and experts. However, vaccine skeptics have pushed back, claiming that they have conducted research and made independent judgments about risks of vaccination based on the evidence.

Public health experts do not argue that vaccination is risk free. Rather, they argue that the risk of vaccination is insignificant compared to the public health benefit from preventing the outbreak and spread of infectious diseases. This is settled science. However, parents who decide not to vaccinate their children are not deciding whether to have their children live in a world without vaccines. Their choice is whether or not to subject their own child to the very slight risk of vaccine-related side effects or instead to make their child a rare exception by refusing vaccination.

Public health research reluctantly acknowledges that rational parents may refuse to vaccinate their kids. This survey looked at parents of intentionally undervaccinated children in San Diego following a measles outbreak in 2008. A child is intentionally undervaccinated when the parent refuses or delays vaccination. The survey revealed that most of the children’s families were affluent, white and college-educated. It’s extremely unlikely that most of these parents were motivated by religious concerns. Vaccine skeptics moderated their views when they considered the possibility of international travel (which would be likely to expose their kids to more infectious diseases). It’s also clear from the survey that these parents had accepted some debunked claims about the risks of vaccination, in particular the possibility that vaccination may cause autism or other adverse reactions. But this doesn’t fully explain why parents decided not to vaccinate.

A second, experimental study of motivations for immunization provides a clue. This study identified altruism, free riding and bandwagon effects as affecting these decisions. This highlights, as the authors note, a clash between public health concerns and individual self-interest. People who accept vaccination not only reduce their own chances of contracting contagious diseases, they also reduce the possibility that they will transmit the disease to others. This “herd immunity” provides a social benefit. Altruistic people are motivated in part by this social benefit when they get vaccinated. However, others see things differently. As the proportion of the population that is vaccinated grows, the individual benefits of vaccination decline, because people are less likely to contract the disease. This makes it more attractive for people to free ride on the vaccination decisions of others. The results of the experiment show that people experience cross-cutting motivations when making vaccination decisions. Bandwagon effects were the most powerful motivating force, suggesting that people are most likely to get their children vaccinated when others around them are doing the same. However, differences in people’s levels of altruism and willingness to free ride also affected their decisions, especially when the issues of vaccination were framed in particular ways.

This clinical report in Pediatrics provides further evidence that parents may refuse vaccination because they are making a strategic decision to exploit herd immunity by free riding on the affirmative vaccination decisions of others. Parents who refuse immunization on behalf of their children are, in a sense, free riders who take advantage of the benefit created by the participation and assumption of immunization risk or burden by others while refusing to participate in the program themselves (p. 1429).

Anti-vaccine parents do not live in a world in which they weigh the risk of infectious diseases versus the risk of vaccination. They live in a world in which their child faces only very slight risks of contracting infectious disease, due to the decisions of all the other socially responsible parents around them who vaccinate their children. This allows them — if they are narrowly self-interested — to avoid the even slighter risk of side effects from vaccination. It is only the socially responsible decision of parents who vaccinate their children that allows anti-vaccine parents to live in that world.

When you put it all together, this evidence suggests that vaccine refusal can be strategic and rational. Many people who refuse to vaccinate their children do not fit the profile of irrational science skeptics; they are affluent and well-educated. In addition, skeptics are more likely to accept vaccinations when there is lower protection from herd immunity (as during international travel). Experimental research shows that people are likely to recognize and exploit vaccination decisions made by others. Clinical advice from pediatricians identifies free riding as a substantial barrier to affirmative vaccination decisions.

Free riders may be anti-social, but they are not irrational. To the contrary, free riders are rationally pursuing their narrow, individual self-interest by exploiting the socially responsible behavior of everyone else. This may, of course, eventually make things worse for everyone. It may also be self-defeating, especially if free riders are clustered together. However, education about the social benefits of vaccines is unlikely to solve the problem of selfishly rational people who refuse to vaccinate their kids. What’s needed instead is government intervention to enforce socially responsible (though not individually risk-free) norms.

Robert Stoker is an associate professor of political science and public policy at George Washington University.