The common argument against the "Group hallucination" hypothesis is that it is impossible for two people to share a hallucination, since hallucinations are projections of the mind; and obviously two people hallucinating the exact same thing is improbable, right? Well- I cringe when I hear apologists say this, since that isn't what the group hallucination hypothesis proposes at all! Actually, the best work on this phenomena states that the hallucinaters experience an "altered state of consciousness", or a type of trance. The trance alone does nothing- it's the hallucinaters that decide what they are seeing- and usually the dominant opinion is the one that comes out on top. They are also not as improbable as we'd think. Bruce Malina and Richard Rohrbaugh's great book, "Social Science Commentary on the Synoptic Gospels" quote this study:
"Erika Bourguignon, who compiled a sample of 488 societies in all parts of the world, at various levels of technological complexity, and found that ninety percent of these societies evidence 'altered states of conciousness' Her conclusion: "Societies which do not utilize these states are historical exceptions which need to be explained, rather than the vast majority of societies that do not use these states" (cited by Pilch 1993)."
For more information, see here. Now, it's important to note that, although these experiences can feel "more real than reality" at some times, the Hallucinaters usually disregard the experience when they are over. However, if we add some pre-Easter prophecies and expectations, it makes sense out of why the disciples would not only believe their experience was genuine, but also re-interpret it the way they did. Lets not forget that the creed is silent on whether Jesus talked or did anything, so it seems likely to me that Jesus just appeared to his disciples and than vanished- just like a hallucination! Now, this fits the data that we have for the group appearance to the twelve, but the appearance to the five hundred seems a bit improbable. Plus, I don't even know where to begin with "the rest of the disciples". So lets consider the appearance to the 500 next.
Well, the appearance to the 500 is a notorious one since it is full of mysteries. For example, the 500 are nameless. William Lane Craig argues that because Paul says "although some of them have fallen asleep", that means he must've known at least some of them personally. I am surprised an otherwise smart man would say this. Maybe he did know someone involved, so what? That hardly means he knew them personally. And even if he did- Paul certainly doesn't share any of the details. We don't know any of the recipients names, we don't know where it happened, or even if they were drinking bad water or something. What little we do know seems to say the opposite! Now, if Paul knew some of them, why would he chose to mention that they had died, rather than name names? He could easily say that the appearance was to "Ralph and 499 other followers" or something like that. Perhaps he says this because the event happened a long time ago (AKA 10-12 years ago), so logically someone would have had to have died by than, considering that people in his day only lived until the ripe old age of 40!
But even if Paul did mention this information, we still have one big problem- it's not mentioned anywhere else in the New Testament! This is prime evangelising material, people! How a story this incredible could evade the quills of the New Testament authors are beyond me. Now, if we consider the "Group Hallucinations" explanation, we know that often, the visions are discredited afterwards. As a matter of fact, this argument is used by apologists like Habermas arguing against the "Group hallucination hypothesis". So with that in mind, it's easy to imagine a scenario in which a large amount of people had an experience that some members considered Jesus. these ones reported it, and the rest discredited it. Eventually the dissenters are heard, and the appearance is later removed from the creed, and thus from scripture (Maurice Casey makes this argument here).
The same can be said of the group appearance to "the rest of the disciples". It's just sad that we don't know how many people were involved. We know that very early Christianity was fairly small, so it couldn't have been that big an appearance (considering a supposed 500 followers already saw him). Still- the fact that they give no number almost seems to imply that it isn't impressive, or isn't even a group appearance at all. At least they added some details to the appearance to the 500, like that some of them died. I'll have to look into it in more detail later, but it seems like at worst it's another massive group appearance that can be explained away in the same manner as the appearance to the 500. Of course, it may not be- but just relying on the text alone doesn't help us much. I'll have to devote a post to this topic in the future.