What is the psychological explanation for mass hysteria?

Factual Questions
1

I was reading article in Slate about a prankster who had three people faint one after another at a Donahue talk show taping in 1985, and this quickly started a round of additional people fainting. “Mass hysteria” has also been cited as the likely mechanism for groups of large numbers of people falling ill in groups if one or more people fall ill very close in time.

This is considered a classic case of mass hysteria.

Another example would be the wildly emotional responses seen when a popular musician or group (the early Beatles fans were noted for this) arrives, and crowds of people (often young girls in these cases) get swept up in an emotional fever pitch.

It seems almost universal among human social groups. The wiki article on mass hysteria is pretty thin and is mostly a list of favorable conditions for it, not why it happens.

What is the psychological engine driving this response? Is it some sort of group empathy we’re wired for as social creatures. What part of our brain wiring predisposes us to this behavior?

2

One relatively recent review in a psychiatric journal concluded:

“The diagnosis of mass hysteria remains contentious, and the mechanisms underlying its perpetuation are similarly ambiguous.”

In other words, “we dunno”.

I haven’t seen any definitive studies differentiating between susceptible and non-susceptible people based on brain chemistry or other factors.

We may be smarter than the average herd beast, but on occasion we behave in nutty ways.