On the first point, I believe the study used surveillance footage so it's likely the majority of people would've been unaware they were being filmed.

Everyone nowadays knows that "we're being watched".

In a situation where bystander intervention might happen, I think people, especially those who are willing to intervene, may not be very conscious about cameras around them. Even more so with surveillance cameras.

> decrease of probability that a particular individual will act, with decrease in group size

I believe the study did show that. I heard an interview with one of the authors on the radio (so my memory may be faulty).

So I don't think it really debunked the bystander effect, rather he made a point along the lines of, 'it doesn't really matter, the more people present means it's the more likely that one of them intervenes'.

That's interesting. I would be really interested in seeing this repeated and plotted as a function of the number of people in the group, up to ~50.

We should also consider that if there's a larger crowd of bystanders, an individual will feel more sure that others will help them as long as they act. "Safety in numbers" so to say.

My first experience I put down to bystander effect:

I was giving first-aid to a guy with severe lacerations to his hand/arm and didn't want to leave him (partially panic on my behalf) - in a UK city, albeit a side-road, with shops and people walking past - but needed to call for an ambulance.

Nobody would stop and help:

"Help! I need you to call an ambulance."

and people just carried on walking.

In the end I was able to makeshift a bandage with some clothing and leave the guy sitting on the floor long enough to use the phone in the shop I was working at.

The ambulance, despite having the address and business name, drove past and I ended up running after it down the street ... overall an awful experience and very eye-opening.

My takeaway is that I'd give up much sooner seeking help from others, depressing as that is, and - contrary to my first-aid training - leave the patient and call an ambulance myself.

Aside: I wonder if there's a villager/townie effect too?

I would guess there's a goldilocks zone. To many and it becomes impersonal so people can walk away. Too few and people don't feel assured of getting back-up?

Do you have research showing that this is a commonly held belief, or that the belief is correct?

Another thing: My understanding of the bystander effect is that it's based on the pop culture understanding of the Kitty Genovese event (see top root comment for real story), which has an additional requirement: The bystanders can't see/talk to each other, but know the others are there in their apartments. The added anonymity is supposed to decrease obligation to help, decreasing the chance anyone will help.