[feanaro]

That would be my guess as well, at least for large N. It might still show interesting behaviour for smaller group sizes, for instance a group of 2 vs a group of 6, or when the probability of a single person helping is comparatively low to begin with (for instance, in a bad neighbourhood known for scams).

The effect is informative regarding what to do when you need someone's help, though. Since the basis of the effect is the diffusion of responsibility, it helps to address a person directly when asking for help. Instead of asking whether anyone in the group can help with something, point to a particular person and ask them whether they can help. In this way, the refusal to help becomes explicit and people will be less likely do that. This is something I've observed from personal experience too.