One of the perpetual puzzlers to the general community of science-and-skepticism oriented folks is the prevalence of people, especially people who are otherwise apparently rational in all respects, who choose not to vaccinate their children. I’m not talking about those that decide their kids don’t necessarily need a chicken pox shot or a human papillomavirus shot; I’m talking about the ones that eschew shots for childhood diseases that used to be both common and deadly, and only aren’t common anymore because generations of children in America have been almost entirely vaccinated against them- polio, diphtheria, pertussis, measles, and so forth.
On the face of a sheer risk assessment calculation, the choice is absurd; the odds of a negative reaction are extremely low and the possibility that vaccines cause autism or some other nebulous lifelong ailment has been shown in twelve studies and counting to be unlikely in the extreme- and the originator of the autism meme has admitted he faked his data. The potential cost with a risk of vaccine reaction is fairly low; among the reactions that DO occur, most are very minor. The potential cost with a risk of a child contracting diseases normally vaccinated against is fairly high; while deaths from such diseases are rarer than they used to be, they can still cripple and scar for life, and such diseases are NOT gone- especially in the age of globalized travel. Skipping the sticks that hundreds of millions of children get with no cost and substantial benefit seems, by actuarial standards, insane. So why do normal, rational people do it?
Because normal, rational people are using brain hardware developed to evaluate the risks of being a hunter-gatherer living in a small stone age tribe rather than being a 21st century member of the developed world- and the brain systems for doing so are fairly jury-rigged even by the hunter-gatherer’s standard. People are not entirely rational* when evaluating risk- and because of the nature of the system, they usually have no idea they’re not unless the disconnect between reason and reaction is glaring enough to get noticed.
For starters, the brain has two entirely different systems for coping with risk- your amygdala is a part of your brain that’s been with you since your ancestors breathed water, and it handles the emotional reactions of risk. This is the part of you that is afraid when it sees a predator nearby, and that teaches you to BE afraid the next time things associated with something Very Bad crop up again. If you’ve ever developed a phobia of something technically innocuous because it was somehow involved in a trauma, thank your amygdala for the reaction. This is how lizards cope with risk and it works just fine for being a lizard.
Meanwhile, we make decisions about risks that are NOT so immediate as a lion coming for your face with an entirely different part of the brain- the neocortex. This is the hot new mammal brain that gives us the ability to do advanced mental tricks like avoid risks that are merely possible rather than ones that are immediate. This is a much harder trick for a brain, and there are all sorts of kludges, shortcuts, and basic heuristics involved in the process. If you want to cover it all read the article I linked and then start in on the footnote material (this is a huge subject), but right now I’ll cover the aspects that directly affect the vaccinate/don’t vaccinate risk evaluation.
-The statistical data for both sides of the issue- the risks of vaccination and the risks of not vaccinating- right now present risks of a size too small to be handled well by our intuitive systems. If you know that if you do something you have a seventy five percent chance of some horrible effect, you’ll never do that thing unless you make some horrible mistake and do it inadvertently. If you know you have a three percent chance of having a horrible result, it will probably vanish off your mental radar altogether unless something else makes you re-consider the risk evaluation- this is one reason why so many people don’t wear sunscreen or don’t fasten their seatbelts. Their costs are almost negligible and the risks are significant, but they feel like it probably won’t happen anyway and the risk is dismissed, nearly automatically. Our brains are designed to cope with numbers relevant to hunter-gatherers- numbers greater than, say, a few hundred are just handled by our brains as “many” or “few”- or “almost never” or “almost certainly”- even with large differences such as one in ten thousand versus one in a million.
-Personalization. We react far more to a perceived risk if we can personalize it rather than thinking about it abstractly. Fifty years ago, the risks of “childhood” diseases were personalized for everyone- basically everybody knew the kid in their neighborhood that had gotten polio and was now crippled, or the kid who was soft in the head after a nasty bout of measles, or the kid who had up and died. Thanks to widespread vaccination efforts, now almost no one who hasn’t traveled to countries where such dieases are still commonplace does; the personalization is relegated to the history books, not “people now like me”. On the other hand, the risk of a vaccine reaction or a possible developmental affect has a personalized face- the kid for whom the power to give the shot or not is in your hands. It is very easy to imagine the child ceasing to breathe or with flat autistic affect. And as every parent knows, risks to your children are always perceived as much more threatening than risks to yourself. This is how a generation went from being kicked out into the neighborhood to play unsupervised for most of the day to playdates with helmets.
-Control bias. Parents see themselves as having a certain degree of control over illnesses that have been with humanity for a long time; they can (they think) keep their child away from people sick with certain diseases and away from hospitals with people sick with exotic things, keep them in with the vaccinated and count on herd immunity to protect them, and if they get sick then we have modern hospitals and modern technology. It’s not the nineteenth century anymore, after all, surely they’re not all that fatal/crippling NOW. Reactions and the imagined potential for poorly-understood developmental disorders are a much less scrutable possibility, with the only sense of control being the choice to vaccinate or not vaccinate- and therefore much more threatening. This is how somebody can ride a motorcycle but still be afraid to fly on an airliner- he’s driving the motorcyle, in control. In a plane his fate is in some stranger’s hands.
-One major factor for risk evaluation is what’s being talked about by people around us. In the developed world, measles and polio epidemics aren’t talked about much anymore, where once they were perennial subjects for all parents. We DO talk about autism and allergies- and regardless of whether they’re being painted as cranks or concerned parents, antivaccination talk is in the air. West Nile virus is still with us at the same rate it was when it was a topic of great concern- a friend of ours recently missed an airport pickup because her ride had been stricken with it- but it’s not talked about anymore, and very few people include it in their risk evaluations concerning activities and precautions anymore. We are a social species- one of our major tools for evaluating dangers is what the rest of the group is concerned about.
-Hidden benefits, hidden costs. The benefit of vaccination is almost entirely hidden- the child is simply not stricken by what is already imagined to be a dismissably-rare event. The cost is usually the same- simply a higher likelihood of a bad outcome. Unless that actually happens- and such cases are usually dismissed as “rare” in the same failure of intuitive numbers-handling- no cost is perceived at all, just as no benefit to the vaccinated child is.
Tying the biased risk evaluation all together is a phenomenon that science fiction writer Douglas Adams once wrote into his books as the Someone Else’s Problem Field**. If you think the risk is going to be borne or mitigated by someone else, but not you (or your child), then its risk profile drops precipitously. What parents who choose not to vaccinate are counting on is every other parent that makes the same risk evaluation differently- and vaccinates THEIR child. This is the aforementioned herd immunity- traditional epidemiologists count on it to protect those who *can’t* be vaccinated or for whom vaccination would make no difference, such as the immunocompromised. If sufficient numbers of vaccinated are present, then diseases will find so few vectors of infection that the outbreak will be a complete nonstarter. Of course, herd immunity depends on very high percentages of vaccinated, so each such choice made breaks the protective wall down a little further- but that’s a remotely abstract concept that the intuitive assessment dismisses, and the individual family is personalized and concrete.
*Link goes to a longer article that explains everything in much more detail. Recommended reading.
**SEP field had nothing to do with risk, but rather was a plot device that allowed the owners of the field to be ignored utterly by anyone around them, but the underlying phenomenon is the same.