I was planning to introduce the topic through a parable of a fictional world carefully crafted not to be directly analogous to any real-world hot-button issues. The parable would be about a hypothetical world where the following facts hold:

• A particular fruit X, growing abundantly in the wild, is nutritious, but causes chronical poisoning in the long run with all sorts of bad health consequences. This effect is however difficult to disentangle statistically (sort of like smoking).

• Eating X has traditionally been subject to a severe Old Testament-style religious prohibition with unknown historical origins (the official reason of course was that God had personally decreed it). Impoverished folks who nevertheless picked and ate X out of hunger were often given draconian punishments.

• At the same time, there has been a traditional belief that if you eat X, you'll incur not just sin, but eventually also get sick. Now, note that the latter part happens to be true, though given the evidence available at the time, a skeptic couldn't tell if it's true or just a superstition that came as a side-effect of the religious taboo. You'd see that poor folks who eat it do get sick more often, but their disease might be just due to poverty, and you'd need sophisticated statistics and controlled studies to tell reliably which way it is.

• At a later time, as science progresses and religion withdraws in front of it, and religious figures lose power and prestige, old superstitions and taboos perish, and now defying them is considered more and more cool and progressive. In particular, believing that eating fruit X is bad is now a mark of bigoted fundamentalism. Cool fashionable people will eat X occasionally just to prove a point, historians decry the horrors of the dark ages when poor people were sadistically persecuted for eating it, and a general consensus has been formed that its supposed unhealthiness has never been more than just another religiously motivated superstition. "X-eater" eventually becomes a metaphor for a smart fashionable free-thinker in these people's culture, and "X-phobe" for a bigoted yokel.

• People who eat X in significant quantities still get sick more, but the consensus explanation is that it's because, since it's free but not very tasty food, eating it correlates with poverty and thus all sorts of awful living conditions.

Now, notice that in this world, the prevailing normative belief on this issue has moved from draconian religious taboos to a laissez-faire approach, while at the same time, a closely related factual belief has moved significantly away from reality. For all the cruelty of the religious taboo, and the fact that poor folks may well prefer bad health later to starving now, the traditional belief that eating X is bad for your health was factually true. Yet a contrarian scientist who now suggests that this might be true after all will provoke derision and scorn. What is he, one of those crazed fundamentalists who want to bring back the days when poor folks were whipped and pilloried for picking X to feed their starving kids in years of bad harvest?

I think this example would illustrate quite clearly the sort of bias I have in mind. The questions however are:

• Does it sound like too close an analogy to some present hot-button issue?

• Does the idea that we might be suffering from some analogous biases sound too outlandish? I do believe that many such biases exist in the world today, and I probably myself suffer from some of them, but as you said, taking concrete examples might sound too controversial and polarizing.