To advocate for anything requires a certain amount of determination, tenacity, and passion. One must be willing to fight for an idea against some other current of belief. Sometimes the beliefs one must fight are deeply entrenched, so activists must expect to hear dissent and, to some extent, expect to reject that dissent. To do so is a necessary strength that maintains a steady course through the winds of change.

Yet there are different sorts of ideas we fight for, with different relationships to evidence.

Take the idea that non-heterosexual or non-exclusive romantic partners are inherently immoral. In examples like LGBTQ rights, the conflict is between two social beliefs: one side arguing that their religious proscriptions should apply to all of society, and the other arguing that everyone should have the freedom to live as they are without discrimination. In such cases advocates have support from the underlying American ideals, and there is no conflicting evidence. Opponents have tried to manufacture that evidence without success, so the conflict remains a social one, and one that LGBTQ advocates are rapidly winning.

In a second category of idea, the evidence for one position is clear, but there are social and economic reasons for pretending otherwise. Climate change falls into this category, and activists can fight to mitigate global warming with a clear conscience. After all, the scientific consensus supports that position. But because the opposition includes powerful businessmen and an entire wing of one major political party, advocates for climate change need to be able to quickly evaluate and dismiss opposing arguments. This isn’t too difficult, because for anyone with scientific literacy and an inquiring mind, the evidence mounted by opponents is clearly cherry-picked, muddled, or fraudulent.

Yet there is also a third, thornier category of idea: that wherein an activist position runs counter to the majority of scientific evidence. For example, there is a vocal minority that fights against vaccines, ignoring the fact that vaccines have been repeatedly proven safe and effective. That minority invents claims at the drop of a hat, seizes on the slightest mention of something the public can recognize as “bad” (like mercury or formaldehyde), and relies on anecdotes and lone retracted papers to counter the overwhelming conclusion supported by literally all the other scientific data.

I find this last category of activist endeavor endlessly fascinating, and I also deeply want to know what it is that leads them to reject the majority of evidence and embrace a position so deeply contrary to the ideal of social change.

I have begun to suspect that what I am seeing is not activism perverted so much as activism taken to an illogical extreme. Advocates for anything need a certain amount of ideological armor to navigate the slings and arrows of outrageous claims, and yet in this last case the fetters of logic have been cast away and the activists themselves have become purveyors of the outrageous. They are become impervious, not just to motivated dissenters, but to whole bodies of objective dissenting evidence.

So too activists must be able recognize and publicize harm that occurs as a result of the opposing view. In the cases of LGBTQ rights and climate change, there are real personal harms that occur from the opposing position. Gay couples are suffering discrimination, and poor coastal countries are suffering unprecedented flooding. Effective activists find these things, drag them into the light, and make society take notice.

In the case of anti-vaccine advocates, though, they rely on made-up harms: the sort of harm one illogically infers rather than the sort of harm with a direct relationship. They make not just unsupported but disproven claims, such as suggesting that vaccines cause autism (they absolutely don’t) or that young immune systems can’t “handle” vaccines (vaccines are less of an immune challenge than almost anything else a child encounters).

Finally, activists need to be able to find and mobilize people who agree with them, and to discredit people who fight against them. When done with the reliable evidence or generally accepted parts of the social contract, such as in the cases of climate change and LGBTQ rights, this is a good and necessary part of creating social change. When done with anecdotes, innuendo, and lies, though, it becomes little more than an ideological cancer. A community of activists can be a center of social innovation, where challenge drives us all to be better, or a hyperbolic chamber of amplified nonsense, where no challenge is ever allowed.

I think, in the end, all advocates and activists walk the knife edge of societal belief, trying to drag that belief farther to one side or the other. This is an absolutely necessary role in society, which might otherwise stay mired in the inertia of bad ideas and the motivated reasoning of the powerful. When activists do this well, they are a check and a balance both on the stagnation of social beliefs. They are nimble, creative, and skeptical of the opposition, but they are also open to new evidence and they embody the ideal of social change.

When activists do this badly, though, they are as brittle and unyielding as the bad ideas and motivated reasoning they so often fight against. The fervor of activism is a part of the solution and a part of the problem both.

I think that strong scientific evidence is the tether holding us on that edge, looking over, and surveying the places we might fall or climb. It lets us reach the edge and innovate, but we must always be cautions to keep close hold of the tether. And, should we find ourselves advocating a position that goes against the majority of scientific evidence, we ought to ask ourselves some very hard questions. If the anti-vaccine advocates are any indication, activists who rush to an extreme relying on bad evidence may fall a long way from the truth, and many never find their way home.