Vaccinations save an estimated 2.5 million lives worldwide each and every year[1].

But a growing number of people are opting not to vaccinate themselves or their children�putting their lives, and the lives of others, at risk[2]. Rates of X, Y, and Z are now on the rise.

What�s behind the growing trend of anti-vaccination? Why do people avoid vaccines that the scientific community has deemed effective[3] and relatively safe[4]?

There are five key reasons.

1. Those who know the least get the worst information

Many people turn to the internet first when they need medical information[5]. Overall, this isn�t such a bad thing�one study found that sources turned up by an internet search offer correct information about 75 percent of the time for everyday, uncontroversial health questions[6,7].

But for vaccination-related searches, the first 100 results were accurate only half the time[6,7]. Accuracy was even worse for common search terms (e.g., �vaccination�) that someone who is less informed on the subject might use. To get the good information, you need to search for technical terms, like �immunization� or �MMR vaccine�[6-9].

2. Some people are less trusting

Some people are more receptive to anti-vaccination messages simply because of their personality. In particular, some people naturally mistrust conventional authority figures[10].

So when governments, large public health organizations, academic institutions, and pharmaceutical companies promote vaccines, they tend to be skeptical�and then turn to less conventional sources of information[10,11].

3. Some people fear contamination

Another personality trait that matters is sensitivity to disgust�essentially, how easily you are grossed out[11-13]. If you are disgusted by the idea of being poked or injected with a foreign substance, you�re going to think twice before getting vaccinated[13,14].

The irony is that vaccines prevent the spread of disease, and all the disgusting things that go with it. But those who are sensitive to disgust see vaccines themselves as more contaminating than the diseases they protect against[11,13].

4. We tend to inflate small risks

Vaccines can have serious side effects in rare cases[15]. But when we hear there�s a 1-in-a-million chance of a severe allergic reaction to a vaccine, we don�t process this information in a cool, rational way.

Partly, this is because the brain can�t fully comprehend just how rare 1-in-a-million is[16]. And partly it�s because we see emotionally vivid events�like having a severe allergic reaction�as being much more likely to happen than they actually are[17,18].

5. We�re wired to find connections and create stories

It�s no coincidence that many anti-vaccination beliefs are about the supposed connection between childhood immunization and developmental disabilities.

Conditions like autism often become apparent around the same age as children are being vaccinated[16]. When a loved one is diagnosed with a medical condition, often our first reaction is to find some way to create a meaningful story to explain it[16,19]. And if our child is diagnosed with autism soon after getting a vaccine that carries �some risk of serious side-effects,� it�s natural to suspect that the vaccine had something to do with it[16].

But it doesn�t even have to be our child. When we hear about a child �developing� autism after getting vaccinated, this sticks in our mind. What doesn�t stick are all the times that children are vaccinated but do not develop any health conditions[16,20-23].

Interestingly, anti-vaccination sources take advantage of these tendencies. While pro-vaccination sources provide statistics describing the small risks, anti-vaccination sources tend to provide emotional stories that shine a bright light on those risks[8,16]. It�s easy to see how the anti-vaccination side could be more persuasive.

References

1. Immunization coverage. (March 2017). The World Health Organization.

2.�Wang, E., Clymer, J., Davis-Hayes, C., & Buttenheim, A. (2014). Nonmedical exemptions from school immunization requirements: a systematic review. American Journal of Public Health, 104, e62-e84.

3.�Measles vaccination has saved an estimated 17.1 million lives since 2000. (2015, November 12). The World Health Organization.

4.�Grammatikos, A.P., Mantadakis, E., & Falagas, M.E. (2009). Meta-analyses on pediatric infections and vaccines. Infectious Disease Clinics of North America, 23, 431-457.

5.�Maglione, M. A., Das, L., Raaen, L., Smith, A., Chari, R., Newberry, S., … & Gidengil, C. (2014). Safety of vaccines used for routine immunization of US children: A systematic review. Pediatrics, 134, 325-337.

6.�Betsch, C. (2011). Innovations in communication: the Internet and the psychology of vaccination decisions. Euro Surveill, 16, 1-6.

7.�Scullard, P., Peacock, C., & Davies, P. (2010). Googling children’s health: Reliability of medical advice on the internet. Archives of Disease in Childhood, 95, 580-582.

8.�Downs, J. S., de Bruin, W. B., & Fischhoff, B. (2008). Parents� vaccination comprehension and decisions. Vaccine, 26, 1595-1607.

9.�Kata, A. (2010). A postmodern Pandora’s box: Anti-vaccination misinformation on the Internet. Vaccine, 28, 1709-1716.

10.�Browne, M., Thomson, P., Rockloff, M.J., & Pennycook, G. (2015). Going against the herd: psychological and cultural factors underlying the �vaccination confidence gap�. PloS One, 10, e0132562.

11.�Clay, R. (2017). The behavioral immune system and attitudes about vaccines: Contamination aversion predicts more negative vaccine attitudes. Social Psychological and Personality Science, 8, 162-172.

12.�Rozin, P., Haidt, J., McCauley, C., Dunlop, L., & Ashmore, M. (1999). Individual differences in disgust sensitivity: Comparisons and evaluations of paper-and-pencil versus behavioral measures. Journal of Research in Personality, 33, 330-351.

13.�Navin, Mark (2013) Disgust, Contamination, and Vaccine Refusal. University of Pittsburgh.

14.�Roser, M. (2017). Vaccination. Our World In Data.

15.�Possible side-effects from vaccines. (2017, May 08). Centers For Disease Control and Protection.

16.�Reyna, V.F. (2012). Risk perception and communication in vaccination decisions: A fuzzy-trace theory approach. Vaccine, 30, 3790-3797.

17.�Tversky, A., & Kahneman, D. (1973). Availability: A heuristic for judging frequency and probability. Cognitive Psychology, 5, 207-232.

18.�Loewenstein, G.F., Weber, E.U., Hsee, C. K., & Welch, N. (2001). Risk as feelings. Psychological Bulletin, 127, 267-286.

19.�Proulx, T., & Heine, S.J. (2009). Connections from Kafka exposure to meaning threats improves implicit learning of an artificial grammar. Psychological Science, 20, 1125-1131.

20.�Nickerson, R.S. (1998). Confirmation bias: A ubiquitous phenomenon in many guises. Review of General Psychology, 2, 175-220.

21.�Lassiter, G.D., Geers, A.L., Munhall, P.J., Ploutz-Snyder, R.J., & Breitenbecher, D.L. (2002). Illusory causation: Why it occurs. Psychological Science, 13, 299-305.

22.�Klayman, J., & Ha, Y.W. (1987). Confirmation, disconfirmation, and information in hypothesis testing. Psychological Review, 94, 211-228.

23.�Chapman, L.J. (1967). Illusory correlation in observational report. Journal of Verbal Learning and Verbal Behavior, 6, 151-155.