One thing I've personally witnessed is people claiming to have had the exact same vivid dream the night before. I'm talking stuff like playing scrabble with Brad Pitt and Former President Carter on the summit of mount McKinley, so it seems unlikely that they were both prompted by the same recent event. Assuming that these people haven't been primed until after the fact, I would expect even stronger effects to be possible for those who have.
Intuitively, I would expect memories of dreams to be different than memories of reality - since dreams themselves are just confabulations in the first place, and memories of them fade rapidly, it's not entirely surprising that people trying to remember their dreams would get confabulations instead.
This suggests a cause for many cases of mass hallucination: sleep deprivation. It's known to cause hallucinations, and in some settings it happens to whole crowds of people at once. Add an ambiguous stimulus and a few people shouting out their interpretation, and you have all the ingredients for a shared hallucination.
Intuitively, I would expect memories of dreams to be different than memories of reality - since dreams themselves are just confabulations in the first place, and memories of them fade rapidly, it's not entirely surprising that people trying to remember their dreams would get confabulations instead.
This suggests a cause for many cases of mass hallucination: sleep deprivation. It's known to cause hallucinations, and in some settings it happens to whole crowds of people at once. Add an ambiguous stimulus and a few people shouting out their interpretation, and you have all the ingredients for a shared hallucination.
I dunno, the Asch conformity experiment seems to suggest that people will doubt the evidence of their senses if enough peer pressure is applied. Combine that with general unreliability of memory formation and maybe you could explain away tens of thousands of people all claiming to have experienced the same weird phenomenon.
I posited the mass hysteria explanation before I read up on Fatima, but I think on balance I still believe more in mass failures of human senses and cognition (which I already know is pretty buggy) than in weird supernatural phenomena.
The asch conformity experiment, it should be noted, makes people say they see a line as slightly longer than it actually is.
Your idea of "otherwise-sane people" seems off to me. "Sanity" is not a binary equation, a switch that you can flip on or off. In fact, you're probably well aware that extremely rational people are susceptible to a lot of the same psychological effect as "the rest". And there is no reason to think that awareness of such effects as mass hysteria is a full-proof shield for it.
In any case, on to your question. A room full of hardcore scientists can still easily be fooled by a skilled magician and if they are not told they are dealing with a magician, they can even be fooled into a "shared vivid hallucination". Worse still, in all likelihood some of these hypothetical scientists and "otherwise-sane people" might be the most prominent defenders of their "observations".
What kind of yield do you expect? What kind of yield do stage hypnotists get? (not that stage hypnotists do light shows, let alone visions, but they're entertaining the audience, not the subject)
There were tens of thousands of people at Fatima. Very few of them claimed to see Mary. When one who did see Mary told Garrett that she wore yellow, Garrett saw a yellow landscape. A lot of people saw a spinning silver disk. That's vivid, but it's not detailed like a person. But how many people is this lot? Do we really know that it's more than 100? 1000? Maybe Garrett went so that he could report if he saw nothing, but mainly these highly-primed people were pilgrims who would be disappointed with themselves not seeing anything.
Does anyone have any evidence that mass hysteria can produce a vivid hallucination shared among multiple otherwise-sane people?
Probably not vivid hallucinations — I'd doubt that mass hysteria could cause people to actually feel as though they are vividly experiencing the thing in question — but vivid false memories, some time after the fact, are much more plausible (and are well-known to be possible, if I remember correctly).
One of the explanations in the irrationality game thread for UFOs and other paranormal events seen by multiple people at once, like the was mass hysteria. This is also a common explanation given for any seemingly paranormal event that multiple people have independently witnessed.
But mass hysteria is mostly known from incidents where people hysterically believe they have some disease, or have some hysterical delusion (false belief). In cases where people report seeing something or having a hallucination, it tends to be a few people across a large society. For example, when reports of Spring-Heeled Jack were going around England, multiple people claimed to have seen Spring-Heeled Jack, but there were no cases of hundreds of people seeing him simultaneously; therefore, the hysteria could have selected for people who were already a little bit crazy, or it could just have been that out of millions of English people a few of them were willing to say anything to get attention.
Conformity pressures can cause people to misinterpret borderline perceptions - for example, if someone says a random pattern of dots form Jesus' face, I have no trouble believing that, thus primed, people will be able to find Jesus' face in the dots. But it's a much bigger leap to assert that if I say "Jesus is standing right there in front of you" with enough conviction, you'll suddenly see him too.
Does anyone have any evidence that mass hysteria can produce a vivid hallucination shared among multiple otherwise-sane people?