In other words, folks left and right need to be very careful when uttering truisms: "Kitty Genovese died because her neighbors didn't care..." "DDT is lethal..." "Women only earn X% of what men do..."
Again and again, these truisms prove to be untrue, but only after they've caused decades' worth of harm.
Unexamined received wisdom can operate like an undetected tumor.
Question everything you think you know. It's not only something we should be telling leftists to do, either.
I've found this to be true time and again. I majored in history in university, and I continue to hold an interest in it, and I have found that our brains are not wired to remember the nuances and details of history, so what we derive and transmit from a given episode or evolution is a nutshell, devoid of context and the background story, that gets passed on and sometimes distorted. (Which is not to say that it's always wrong!)
Kathy Shaidle spoke of the issue of the purge of the Birchers from the conservative movement. I'm not familiar with that particular episode. But I've seen it in various narratives of history, particularly in Catholic Church history.
We tend to blame anti-Catholics for getting it wrong, and making stuff up. Faithful Catholics sometimes do it too- not maliciously, of course. It has to do with how we remember and transmit history in the vernacular. It's not a conspiracy. It's not an attempt at deception. It's just that we can't remember the whole story, particular in this day and age when we don't have oral history, and we downplay rote memorization.
The Spanish Inquisition is a topic I've read a good deal about. But don't ask me to recall the finer points of Henry Kamen's book on the subject. I know the facts that I like to bring up in debates on the issue, but I really couldn't give you a precise rundown on the historiography. We tend to forget what we learn because we're not made to remember huge amounts of interconnected facts on a large number of topics. Maybe some individuals have photographic memory, but those are the exceptions.
This is why I sometimes remain silent on salient points of the day. Does abortion lead to breast cancer? I'm inclined to believe it, but I can't say. Is Islam inherently violent? I can't say. Is climate change a fact, and is it man-made? Beats me.
Who has time to settle all these debates. In my world, whether abortion causes breast cancer, whether Islam is inherently violent, whether climate change is man-made is just not relevant.
I'm comfortable with not the truth, or even an opinion, on everything.
There are only so many hours in a day. It might be nice to have an ideology that tells you what to think on everything, but I just don't operate that way. When I have an opinion, it's because I feel sufficiently confident of the basis for it. And even then, some opinions with more conviction than others.