Understanding that vaccines do not cause illness

Jessica Glass Kendorski, Ph.D., NCSP, BCBA-D

Vaccination—the word has become quite divisive. On both sides, we find well-meaning, intelligent, good parents who just want what is best for their children, and to protect them from harm. The science behind the benefits of vaccinating are clear (see this Healthy Kids blog about the importance of herd immunity), and the recent measles outbreak shows the consequences when individuals do not vaccinate. The total number of cases is up to at least 155 cases in 16 states. Over the weekend, New Jersey became the latest state to join the list.

So why are well-meaning, intelligent, parents, putting themselves and their children at risk? Part of the reason is because many people have not personally experienced the devastation of some of these viruses for which we now have vaccines, causing them to underestimate the seriousness of illnesses like measles. But there’s another factor in play: a natural, predictable bias in the way humans are wired to think, a bias known as the post hoc fallacy--the natural tendency for us to see causation in two events that co-occur in time.

The post hoc fallacy, fully named “post hoc ergo propter hoc” translates from the Latin “after this, therefore because of this.” It is an error in logic to assume that if event 1 occurs right before event 2, then event 1 necessarily caused event 2. As a personal example, my daughter received her booster MMR vaccination when she was 4 years old. The very next day she developed a rash and spiked a 104-degree temperature. I was scared, but, fortunately, it was clear to me that this probably wasn’t because of the vaccination; her 2-year old brother had had the same rash and high temperature a few days prior, and had been diagnosed with the Hand, Foot and Mouth virus. It would have been natural for me to blame the MMR vaccination for her illness (since one occurred right before the other), but I had the “benefit” of having another kid in the house with similar symptoms who 1) had been diagnosed with a highly contagious illness, and 2) had not been recently vaccinated.

Young kids get sick with regularity.  Young kids get vaccinated with regularity. Therefore, there is a reasonable chance that your child will get sick some time after receiving a vaccine, without the two events being related.  Similarly, with Autism Spectrum Disorder, symptoms begin to emerge around the same time that toddlers routinely receive the MMR vaccination. This timing predictably causes some parents to question whether the vaccination caused the disorder, when there is no evidence that suggests the two are related, and substantial evidence supporting that the two are unrelated.

This tendency to equate causality with timing has long been shown in the behavioral science literature. Take B.F. Skinner’s experiment with pigeons. The birds would receive a food pellet at random intervals from a machine. After a certain amount of time, they would perform whatever random activity they had done before receiving the pellet, as if that action would produce it. This is exactly why humans strive to understand what happened —what they did—to cause an event. And, it’s the reason why so many parents are convinced that a particular vaccination can cause an injury or disorder, when in reality, the two are unrelated.

If we want to increase the number of people who vaccinate their children, explaining the science is not enough. We must understand the natural tendency for people, all people, to equate coincidence with causality. We must recognize the stories that people tell about their child getting sick are very real, and very heartbreaking. We must help people understand the fallacy in their logic; that just because two things occur together does not mean that one caused the other. Any successful message has got to include real empathy.  Empathy for these very, well-intended parents, who have only the best interest of their beloved children at heart.