The science is clear. There is no evidence that childhood vaccines cause autism spectrum disorder. There is evidence that vaccines do not cause autism. For example, rates of autism spectrum disorder do not differ among children who have and have not been vaccinated. There is evidence that vaccines prevent potentially deadly diseases, such as measles and pertussis. There is evidence that unvaccinated children are at increased risk for contracting diseases and can spread infection to vulnerable others, such as infants who are too young to get immunizations. If the risks of not vaccinating are coldly compared to the risks of vaccinating, vaccination wins without question.Yet more than one in ten parents do not follow the recommended vaccination schedule because of concerns about the dangers and necessity of immunizations. Among those parents who adhere to the vaccination guidelines, one in four say they are not sure that is it good for their children. Even more puzzling is that providing messages about the safety of vaccines and the dangers of vaccine-preventable diseases to parents who are afraid of vaccinations makes them less likely to want to get their children immunized. And because of a small but growing number of individuals who are not vaccinated, local outbreaks of measles—a disease that was declared eliminated in the United States in 2000—are no longer uncommon.If a Vulcan researcher were studying parenting behavior on earth, the increasing frequency of mothers and fathers who refuse to vaccinate their children would look completely irrational. But if you look at this behavior through an evolutionary lens, it makes perfect sense. Humans are born to make bad decisions.How It All StartedA study published by Andrew Wakefield and his colleagues in 1998 in the journal The Lancet linked the MMR (measles, mumps, rubella) vaccine with autism. Soon growing numbers of parents began to notice that shortly after getting their children vaccinated symptoms of autism appeared. The conclusion: “The vaccine must have caused the autism.”Since then, the 1998 study has been thoroughly discredited and retracted from the journal. Wakefield has been banned from practicing medicine and found by the UK General Medical Council to have acted “dishonestly and irresponsibly.” And not a single researcher has been able to find a causal link between autism and the MMR vaccine.The lack of any science associating vaccines with autism hasn’t stopped as many as one in four people from believing in the vaccine connection. Nor did it stop US senators John Kerry, Chris Dodd, and Rand Paul from trumpeting the idea that vaccines cause autism. It didn’t stop Robert F. Kennedy Jr. from writing a Rolling Stone article accusing the government of protecting drug companies by concealing evidence that mercury in vaccines likely caused autism in thousands of children. It hasn’t stopped celebrity Jenny McCarthy and her three bestselling books from blaming the MMR vaccine for giving her son autism.These stories hold power despite contrary facts and even in the face of rising rates of preventable diseases due to dropping immunization rates. Is this a problem? It is only if it worries you that the risk of an infected infant dying of pertussis is about one percent, but the risk of an infant dying from the pertussis vaccine is practically nonexistent. This fact makes Jenny McCarthy more a threat to public health than any vaccine.Our Evolutionary BaggageMuch of the conversation in recent weeks has blamed non-vaccinating parents for being ignorant, stupid, or even worse. But calling someone stupid does not seem like an effective way of getting parents to agree to do something that they believe might harm their child.Another way to look at parents’ fears about the risks of vaccinating is to look at decision making through an evolutionary lens. Are parents, including those who choose not to immunize their children, really irrational decision makers when it comes to weighing risks? Maybe not. Maybe their decisions only seem irrational because we are not looking at human behavior in our species’ natural environment. Rather we are looking at human behavior in a newfangled modern environment that is jammed with things like tentative science, sophisticated statistics, erroneous online information, and sensational media reports. These are all evolutionary novelties to which our brains have not yet had the time to evolve effective responses.For most of human history, our decisions about risk were based on our own observations or those of trusted others of everyday life: “Junior ate that yellow plant by the marsh this morning and got sick before bed. That plant must have made him sick and so that plant is dangerous.” This tendency to link co-occurring events likely was a very effective strategy for our ancestors because false positives generally are harmless (e.g., believing that there is a connection between a yellow marsh plant and the plague, when there is not), whereas false negatives (e.g., believing that there is no connection between a yellow marsh plant and the plague, when there is) may take someone out of the gene pool.This tendency to causally link observed phenomena that occur close together in time remains with us today. It makes us ignore the Statistics 101 mantra “correlation does not imply causation.” It makes us think that symptoms of a certain neurodevelopmental disorder that develop roughly around the same time as a certain vaccination must be causally related. In other words, the notion that vaccines cause autism and other disorders emerged and still persists because symptoms of autistic spectrum disorder tend to appear around the same time as when the MMR vaccine is given—during the second year of life.There also is evidence that when it comes to many sorts of decisions, including weighing the risks of childhood vaccines, we tend to trust our friends and even the media more than we trust experts, like health practitioners and research scientists. What’s so bad about experts? Experts are evolutionary novelties and we are not sure what to do with them. As such, it makes sense that parents would be more likely to believe a neighbor or a mom-blog who is certain that her son got autism from a vaccine than some professional who they have never met and who probably is much too busy to have children of his own. Historically, it probably was way more risky to ignore advice from a peer than to take it.So from the perspective of our ancestors, a bias to believe trusted others likely made much sense. However, our distrust of scientists can be problematic in the modern world where so many of our decisions can be informed by science: Should I place my baby to sleep on her back or on her side? Should I breastfeed or bottlefeed her? Should I make my older child wear her bicycle helmet? Is it safe to feed her GMOs? As one illustration of our tendency to distrust scientists, we know that people are more likely to report belief in ESP if they are told that the majority of the general public believes in ESP, and that they are more likely to doubt the reality of ESP if they are told that the scientific community accepts ESP. This is absurd. Shouldn’t people be more likely to believe experts than a bunch of regular Joes? Does this mean that people automatically are wary of scientific findings? It means that scientists cannot just pile on the “facts” about the safety of vaccines and the health dangers of not vaccinating and expect it to take hold. They have to do more.Fear also muddies our decision making in the face of facts. Generally our decisions are more easily compelled by fear than assurance. Objectively speaking, decisions should be made by an evenhanded appraisal of risks versus benefits, but emotionally we are more influenced by the potential for danger than for gain. We are more swayed by the (discredited) fears raised by Wakefield than the consistent assurances from the scientific literature that vaccines are safe. We are more affected by a moving story than statistics even if there is only a remote chance that it will ever happen to us. This is why we are out of the water the moment we hear a theatrical story of shark attack (despite the fact that humans kill more sharks than sharks kill humans and that humans are more likely to be bit by another human than by a shark).We are compelled by fear because it comes from a super-fast risk assessment driven by primitive parts of our brain. Our brain flags certain perceptions, sends an alarm signal, and—before we’re even aware of it—floods our brain with adrenaline. This is the way our ancestors assessed risk before we had statistics. So we’re built to gauge fear reflexively, before the entirely of it all is consciously perceived—not to first consult the scientific literature, email the experts, Google the risks, confer with friends, think it through, and then, and only then, determine if we should be afraid. If our species carried on like this on the savannah, humans would have gone extinct long ago.There is also the power of our preexisting beliefs. If we already believe something to be true, it is very hard to get us to believe otherwise. Even when we try to be completely objective, we are constrained by what we already think. We filter incoming evidence in ways that support our existing views and ignore or reinterpret any evidence to the contrary.These tendencies happen because of the way that our memories have been designed. When we do a Google search, the computer systematically searches all relevant data. When we search our minds, we more easily find evidence that confirms the things that we already believe in. For example, when the Surgeon General’s 1964 report on smoking was released stating that smoking causes lung cancer smokers were less persuaded by the report than nonsmokers. Smokers used all sorts of anecdotal evidence to counter the statistics, like: “I know a lot of smokers who haven’t been sick a day in their life.” Or they reasoned away the dangers: “Drinking is worse than smoking” or “Smoking is better than being fat.” These sorts of findings give us every reason to believe that parents who already think that vaccines are dangerous filter out messages that convey the safety of vaccines and consequently settle more tightly into existing views when they encounter information that challenges their belief. So those of you reading this who are wary of vaccines—you are more likely than others to find holes in my arguments, argue that you could be doing much worse, or think that I’m some sort of quack.This autism/vaccine story is a potent example of why it is important for us to be good consumers of science and to understand some of the biases in our thinking that we have developed over evolutionary time. The modern world is stuffed with groundless claims, baseless opinions, affecting anecdotes, bad science, and elegant research. With our brains marinating in all sorts of ancient influences, making good decisions about how to best keep our children safe and well can be tough. Our brains can sucker us in to believing things that simply are not true and that in fact can be harmful. However, the more we know about the evolved tendencies of our brains, the better fit we are to smooth out the rough spots that evolution has left.