In this multi-part blog we’ll examine the cognitive processes involved in deciding to vaccinate a child.  Specifically, I want to answer the question, “what prevents people from arriving at the truth?” It’s important to note that I presuppose the truth about vaccines has been established.  I take it for granted that vaccines are beneficial AND safe. I’m in good company with this assumption as the vast majority of the scientific community, doctors, and even parents agree1. I am not interested in debating specific claims about vaccines themselves.  If you are in the vaccine-refusal crowd, I would like to refer you to the incredible amount of reliable resources available from accomplished scientiststrusted pediatriciansand world-renowned hospitals.  My purpose here is to discuss why, despite very strong evidence that vaccines are safe and almost-miraculously effective, people put their children’s health at risk by declining to vaccinate them.

 Vaccines.  Are.  Awesome. 

In order to make any medical, financial, or life decisions, one must first examine the benefits of the choice.  If a parent consents to allow their child to be stuck by a needle and injected, it better be for a good reason.  So what are the benefits of vaccines? This question is both surprisingly easy and oddly difficult to answer.

The easy answer: vaccines can save your child from dying of a horrible, vaccine-preventable disease (VPD).  As an example, these potential deaths include but are not limited to:

  1. Liver failure (hepatitis B)
  2. Dehydration from diarrhea (rotavirus)
  3. Respiratory failure from chocking on snot (pertussis)
  4. Respiratory failure from full body contractures (tetanus)
  5. Respiratory failure from choking on bacterial goobers in the throat (diphtheria)
  6. Respiratory failure from invasive bacterial disease (pneumococcus)
  7. Respiratory failure from a swollen epiglottis (h. flu)
  8. Respiratory failure from paralysis (polio)
  9. Brain failure [encephalitis] (measles)
  10. Meningitis (meningococcus)
  11. Cancer (HPV)
  12. Smallpox

…and this is not an exhaustive list nor does it include the multitude of complications each of these cause.

Let me highlight these last two.

First, smallpox.  Smallpox killed an estimated 300-500 million people in the 20th century (see this awesome graph above). Do you know anything about smallpox? No? Me neither! And I’m a doctor! I know nothing about it because it no longer exists on earth except in laboratories.  And we have vaccines to thank.  Through wide-scale vaccination campaigns, endemic areas were targeted and the disease was eradicated from the world.  I invite you to sit back for a minute and marvel at that thought.

Now Let’s talk about the human papillomavirus (HPV) vaccine.  Through the use of a shot, we can prevent cancer, that old Emperor of All Maladies. HPV infects the cervix of women and causes inflammation, which over many years promotes cancerous changes in the cervical cells.  Worldwide, cervical cancer is the fourth leading cause of death from cancer for women, causing about 260,000 deaths per year, or 1 in 14,000 females. And every single one of those deaths is preventable.  With the vaccine we can prevent the cancer from ever forming.  Think about it: no chemotherapy, no needing your uterus removed or your abdomen surgically explored for lymph nodes, no risk of cancer, all because 4 decades earlier you were vaccinated.

So there are significant benefits from vaccines.  Everyone enjoys not dying from horrible diseases, even those who are anti-vaccines.  Given the benefits, why not vaccinate? The answer lies not in what the facts are, but in how the human brain evaluates them.

Vivid Outcomes

If you thought the section above was mildly graphic, it was intentional.  Research demonstrates that the more affect laden an idea, the more detailed and vivid with emotional content, the more likely the human brain is to pay attention to it.  Advertisers, journalists, and Buzzfeed have long known this fact and it is now wreaking havoc on every form of social media: “I thought it was just a 3-armed homeless raccoon until 1:37 and then waterfalls flowed from my eyeballs.”

Vividness makes a fact more real to the brain.  To demonstrate this, researchers measured the amount participants would pay to avoid a chance of receiving a small electric shock2.  The probability of the shock varied between participants, with some having low probabilities of being shocked and some having high probabilities.  Those who faced a 1% chance of a shock were willing to pay $7 to avoid the shock, which was proportionally more compared to those who faced a 99% chance and were only willing to pay $10.  In other words, the factor that most influenced the price was the mere possibility of a shock, while increasing probability was comparatively undervalued.  Electric shocks are vividly unpleasant.  The fact that the probability existed, however small, was enough to make it real in the minds of the participants and worth paying to avoid.

Our perception of statistics is also subject to vividness. Above, I gave a concrete number for cervical cancer deaths, 1 in 14,000 females.  I could have expressed it as 0.00007% of all women; however, the human brain is terrible with such percentages, which are translated as “negligibly small”.  In the former representation, people focus on the 1 person and tend to ignore the denominator of 14,000, making the chance seem much larger3.  Comparatively, 1 person at risk is much more compelling than a difficult-to-visualize small percentage of people.

Now, this explains why I described the benefits the way I did, but it does not explain why people do not vaccinate.  The issue is that while the benefits can be quite vivid, they are not on people’s minds.  Part of this is the fault of the success of vaccines.  Few people living today know the terror of smallpox, so in absence of obnoxious descriptions of its evils, it has lost its vivid force.  The same goes for the more relevant VPDs like measles.  When infection rates fell, measles faded into the background of parental worries because few people had to watch their child suffer from it.  Inevitably, people started questioning the benefit of the shot because the threat was no longer real to them.  Unfortunately, measles does not care how real you think it is.  So the benefits lose their affective power when they are not salient issues in people’s minds.  Of course, it can become real again as the recent affect-rich measles outbreak at Disneyland demonstrated with its flood of media attention and angry Facebook statuses.  It is my prayer/hope that more kids do not have to become affect-laden images, but given how quickly we forget current events, it seems that will inevitably be the case.

Immediacy Effect

People also evaluate benefits by how long it takes to realize them.  As a general rule, we tend to value short-term benefits more than long-term benefits, a phenomenon researchers call the immediacy effect4.  Studies consistently show that when faced with the choice, people will choose a smaller denomination of money if they receive it sooner compared to a larger sum paid later.  The short-term holds more promise to give you pleasure in the time that actually matters, the present.  Furthermore, the short-term has a greater chance of actually happening compared to the long-term, where there are more chances for a hope to go unfulfilled.  We experience this effect every day in the form of procrastination: “I should do my taxes that are due in 3 months but I’ll probably just sit here and watch this video of goats yelling like humans instead.”  The future deadline of taxes holds little value compared to the hilarity of those goats.  Similarly for vaccines the benefit of avoiding a disease is sometime in the future, but the pain of watching your child receive a shot will happen right now in front of your eyes.  Depending on how the parent evaluates the situation, she may value avoiding tears now more than avoiding a disease she’s never seen before at some uncertain time in the future.

Ambiguous Probability of Benefit

While people may acknowledge that avoiding VPDs is generally a good goal, a parent may dispute the probability that he or his child is in any appreciable danger from them4. The benefit of vaccines is easily demonstrable on a population level; however, at an individual level a person may perceive that his risk is minimal.  If you couple this doubt with a perception of increased risk from the vaccines themselves, then the scales will tip strongly in favor of forgoing the shots.  The fallacy of this thought is that, while certain diseases like polio occur very infrequently, if this mindset was prevalent throughout the population, then your risk profile changes as the disease becomes more prevalent (e.g Parents at Disneyworld thought the chance of their child contracting measles was basically zero).  Statistics of disease rates are a slippery measure, as decisions made on such statistics will change the rates themselves.

A related situation to frequency is when parents doubt severity of the disease.  A very popular argument against the measles vaccine is that…measles schmeasels, amIright!  In other words, it’s a fever, cough and rash, big deal.  This is based on a popular view of how the disease manifests but fails to acknowledge the epidemiological data.  All diseases present in spectrum of severity and measles is no different.  One in 1000 kids with measles will die from it in developed countries, and if you’re vitamin A deficient in a developing country, it could be 1 in 4.  When you take into account the other complications, days hospitalized, sick days from school or work, then you can see how easily measles becomes a big economic and public health burden in areas with low vaccination rates.

I know I said I wouldn’t slog through the weeds of specific arguments, but these are illustrative of a larger point.  The human brain can very easily 1) manipulate statistical data for its own purposes and 2) downplay statistical data if a representative mental image is present.

Affective Reasoning

It’s a peculiarity of human reasoning that, despite the lip service we give to rationality, our judgments can be partly or entirely based on how we feel toward the topic.  Cognitive psychologists call the role that emotions play in reasoning the affect bias5.  Research shows that negative feelings towards vaccines will cause someone to underestimate potential benefits while overestimating potential risks.  These feelings do not need to be based on data.  A negative anecdotal story about “vaccine injury” on the Internet might change perceptions more than a ratio of numbers on paper because the story makes us feel scared while the numbers make us feel tired.  What’s more, our general feelings, unrelated to the topic at hand, can also affect our judgment.  We will discuss the affect bias and risk assessment at length next post, but here let it suffice to say that if we feel negatively towards vaccines, we are more likely to undervalue its benefits.


So vaccines are awesome and their primary benefit is not dying from VPDs.  People evaluate these benefits along certain parameters.  Vividness and affect-laden imagery make people perceive benefits as more real.  Because VPDs are rare, they are not salient in people’s minds and thus they lack vividness. People also value short-term benefits over long-term benefits.  Because preventing VPDs are a long-term benefit, they are undervalued compared to the short-term benefit of avoiding a child’s discomfort.  People may doubt the need for the benefits because many people will never suffer from VPDs. These rough statistical evaluations focus on isolated cases while ignoring the wider context of risk that may increase because of such decisions.  Our feelings may shape our decisions regardless of any benefit that exists.  Affect bias is a decisive yet oftentimes invisible factor in our judgment process.

Those of us on the pro-vaccination side may have difficulty understanding how the clear benefits of vaccines can be ignored by the anti-vaccination crowd.  However, we can understand that a statement of fact does not imply a conclusion.  The fact has to survive a mental obstacle course before it has meaning to a human brain.  We may not agree with their conclusion but we can appreciate the process because we go through it as well.  Moreover, we can understand that a statement of benefit does not determine an appropriate choice.  People who smoke, ride their bikes without a helmet, or choose to watch cat videos instead of studying for an upcoming test know this tension well.  For vaccinations specifically, benefits are only part of the story.

Risk, the flipside of benefits, plays a powerful role in decisions. In the next post we will examine the human brain’s relationship to risk.  The relationship is complicated, filled with so many emotions and selfishness it deserves its own reality show.  Most importantly, beyond demonstrating how we make health decisions, our relationship to risk illuminates an aspect of our mental life that is central to what makes us human.


  1. Smith PJ, Humiston SG, Marcuse EK, et al. Parental Delay or Refusal of Vaccine Doses, Childhood Vaccination Coverage at 24 Months of Age, and the Health Belief Model. Public Health Reports. 2011;126(Suppl 2):135-146.
  2. Rottenstreich Y, Hsee CK. Money, kisses, and electric shocks: On the affective psychology of risk. Psychological Science. 2001;12(3):185-190.
  3. Kahneman D. Thinking, fast and slow. Macmillan; 2011.
  4. Cappelen A, Mæstad O, Tungodden B. Demand for Childhood Vaccination – Insights from Behavioral Economics. Forum for Development Studies. 2010;37(3):349-364.
  5. Slovic P, Finucane ML, Peters E, MacGregor DG. Risk as analysis and risk as feelings: Some thoughts about affect, reason, risk, and rationality. Risk analysis. 2004;24(2):311-322.
  6. Finucane ML, Alhakami A, Slovic P, Johnson SM. The affect heuristic in judgments of risks and benefits. Journal of behavioral decision making. 2000;13(1):1-17.
  7. Lichtenstein S, Slovic P, Fischhoff B, Layman M, Combs B. Judged frequency of lethal events. Journal of experimental psychology: human learning and memory. 1978;4(6):551.
  8. Medicine Io. Adverse Effects of Vaccines: Evidence and Causality National Academies Press 2012.