Yet these fears, fierce and visceral, persist. To frustrated doctors and health officials, vaccine-phobia seems an irrational denial of the facts that puts both the unvaccinated child and the community at greater risk (as herd immunity goes down, disease spread rises). But the more we learn about how risk perception works, the more understandable — if still quite dangerous — the fear of vaccines becomes.

Along with many others, the cognitive psychologists Paul Slovic of the University of Oregon and Baruch Fischhoff of Carnegie Mellon University have identified several reasons something might feel more or less scary than mere reason might suppose. Humans subconsciously weigh the risks and benefits of any choice or course of action — and if taking a particular action seems to afford little or no benefit, the risk automatically feels bigger. Vaccinations are a striking example. As the subconscious mind might view it, vaccines protect children from diseases like measles and pertussis, or whooping cough, that are no longer common, so the benefit to vaccination feels small — and smaller still, perhaps, compared to even the minuscule risk of a serious side effect. (In actuality, outbreaks of both of these infections have been more common in recent years, according to the Centers for Disease Control and Prevention.) Contrast this with how people felt in the 1950s, in the frightening days of polio, when parents lined their kids up for vaccines that carried much greater risk than do the modern ones. The risk felt smaller, because the benefit was abundantly clear.

Professor Slovic and Professor Fischhoff and others have found that a risk imposed upon a person, like mandatory vaccination programs (nearly all of which allow people to opt out), feels scarier than the same risk if taken voluntarily. Risk perception also depends on trust. A risk created by a source you don’t trust will feel scarier. The anti-vaccination movement is thick with mistrust of government and the drug industry. Finally, risks that are human-made, like vaccines, evoke more worry than risks that are natural. Some parents who refuse to have their kids vaccinated say they are willing to accept the risk of the disease, because the disease is “natural.”

Still, shouldn’t our wonderful powers of reason be able to overcome these instinctive impediments to clear thinking? The neuroscience of fear makes clear that such hope is hubris. Work on the neural roots of fear by the neuroscientist Joseph LeDoux of New York University, and others, has found that in the complex interplay of slower, conscious reason and quicker, subconscious emotion and instinct, the basic architecture of the brain ensures that we feel first and think second. The part of the brain where the instinctive “fight or flight” signal is first triggered — the amygdala — is situated such that it receives incoming stimuli before the parts of the brain that think things over. Then, in our ongoing response to potential peril, the way the brain is built and operates assures that we are likely to feel more and think less. As Professor LeDoux puts it in “The Emotional Brain”: “the wiring of the brain at this point in our evolutionary history is such that connections from the emotional systems to the cognitive systems are stronger than connections from the cognitive systems to the emotional systems.”

And so we have excessive fear of vaccines. But just as we are too afraid of some things, this same “feelings and facts” system works the other way too, sometimes leaving us inadequately concerned about bigger risks. A risky behavior you engage in voluntarily and that seems to afford plenty of benefit — think sun-tanning for that “nice, healthy glow” — feels less dangerous. A societal risk, well off in the future, tends not to trigger the same instinctive alarm — in part, because the hazard isn’t singling any one of us out, individually. This helps explain why concern over climate change is broad, but thin.