Someone had a thread elsewhere denouncing pseudoscience; it got me thinking, what are some historical examples of pseudoscience going mainstream? There must be some.
Oddly the one that comes to my mind is the idea that stomach ulcers are caused by an infection. Loudly poo-pooed, it is now accepted.
There must be other, better examples. I sure cannot think of any.
Why do you consider that example pseudoscience, as opposed to being merely an idea that didn’t fit in the current paradigm?
To my mind, if a theory is truly pseudoscience, then the only way it will be true is by coincidence - finding the right answer by the wrong methodology.
Is any theory which is thought not to be true by most scientists for some period of time “psuedo-science”? That seems to be the criteria your using, but I don’t think it fits what most people think of as psuedo-science.
Psuedoscience is the practice of using other-than-scientifically-rigorous methods to reach a conclusion. Did someone use non rigorous methods to deduce that infections cause stomach ulcers? If not, then it’s not pseudoscience.
Tectonic plate geology may fit your question. If I recall correctly Wegener’s ideas on continental drift were not only “poo-pooed” but ridiculed by geologists for decades until later proved.
That wouldn’t be pseudoscience at all to my way of thinking. The bacterial cause of stomach ulcers was a theory well within mainstream scientific thinking and practice - it just wasn’t given much credibility in the face of other explanations. It’s a classic example of a shunned theory eventually proving to be correct, but it’s not pseudoscience.
I can’t think of any psuedoscientific theory that was accepted in mainstream science in modern times - it’s sort of a contradiction in terms. There’s plenty that have become accepted in mainstream thought in general of course, outwith science. If you go back hundreds of years then things like the development of alchemy into chemistry might fit the bill - trial and error pseudoscience slowly becoming scientific as understanding in other disciplines developed.
The first thing that comes to mind for me is the theory of continental drift. The basis for the theory was essentially the observation that the continents can be seen to fit together (such as South America and Africa).
However, the theory was not given much credence by mainstream science, because nobody had any idea how continents could plow through solid rock. At the time that Wegener presented his work in the 1910s and '20s, his work was widely derided and surely put into the category of “pseudoscience” by the mainstream establishment, because Wegener had no plausible mechanism to explain his observations.
Not until the theory was refined into the modern theory of plate tectonics did the theory become mainstream, and this was not until the 1960s.
ETA: And I see that I was beaten to the punch while writing this. See what I get for writing such a detailed post? 
This thread is going to go nowhere until everyone accepts that pseudoscience does NOT mean “an idea that many scientist didn’t believe”. Continental drift and ulcers are not and have never been pseudoscience, but are always inevitably the first examples brought up in this type of thread.
Maybe I misunderstood the Op. but what about chiropracty? According to “Fads and Fallacies” which was published in the '50’s, chiropracty was the worst kind of semi-holistic claptrap ever practiced, but today everybody acts like chiropractors are just like regular doctors. Don’t get me, wrong for all I know chiropractors may have gotten rid of all that snake oil, but it was still considered bunk as late as the '50’s.
Everybody does not act like that. Many doctors feel chiropractors are not real doctors because the science does not support it.
Adding to the point made by Speak to me Maddie and robby, Wegener’s ideas were not only ridiculed but ridiculous – the idea that the comparatively light and fragile continental rocks might hold shape while floating on the denser, stronger oceanic crust was absurd. Continental drift as proposed by Wegener was very much pseudo-scientific, an explanation of coincidental similarities in coastlines that was not supported by geophysics and geotectonics of the time.
With plate tectonics, though, what had been a pseudo-scientific idea was revised – the ocean crusts were produced by upwelling, forcing plates on which the continents rode as if passengers apart. , with plate collisions and the resultant subduction producing island arcs and continental-edge orogenies. This was solid geophysics.
So it is a near-perfect example of what’s being talked about – an idea ridiculed for good reason that proves out to be correct for theoretical reasons not advanced by the person originally advocating the ‘ridiculous’ idea.
I’d second the idea of Continental Drift, as one of the few modern examples of a genuinely kooky idea, that would certainly be described as “pseudoscience” at the time, and is now part of the mainstream. As opposed to most scientific theories which at one time or other were the dissenting opinion of a minority of scientists.
Please do not include me (or most of my friends) in “everybody”.
Wegener was a mainstream scientist who used scientific methods and observation to piece together information. He traveled to the various sites he talked about to study their geology. He studied the fossil records of continental edges. At the end he put together a full record of his researches. He postulated a mechanism that couldn’t be substantiated at the time.
What did mainstream science give as an answer for the many correspondences they knew existed? Land bridges. Why are they being given a pass for a mechanism that had as serious flaws in it?
The better way to look at history is that Wegener was right to insist that the correspondences required one-time proximity and the mainstream was right to insist that a mechanism plausible and workable by the science of the day be required. Neither had enough information to give a full answer in the way we understand today but both were doing science.
Chiropracty, conversely, has always used a principle that is entirely unrecognized and undiscovered by mainstream science, subluxations.
Chiropracty was, is, and will remain snake oil as long as it adheres to its core concepts.
That’s the difference between science and pseudoscience. Pseudoscience starts with concepts wholly outside of science, doesn’t bother to test them, insists on their truth despite foundation, and claims persecution when challenged.
That’s why there are no pseudosciences that ever become science. Lack of knowledge can be cured by time and investigation. Militant belief can only become narrower.
Does that mean this is a pseudothread?
Freudian psychology was fairly well pseudoscience. It has now been largely cast aside, but for decades it was practiced as “science”. And of course the same thing with almost all medical knowledge up until the late 19th century.
Isn’t this the exact opposite of what the OP is asking for?
Antioxidants?
What was ever pseudoscience about antioxidants?
There is definitely a lot of solid science supporting antioxidants asGood Things For You, though that’s hardly a settled matter. There’s also a good deal of science out there saying that antioxidants by themselves aren’t particularly helpful.
However, antioxidant supplements are sold with over-broad claims of health benefits (to be charitable about it), in a manner that’s definitely pseudoscientific. The alternative health movement have seized on antioxidants as cure-alls.