  The realism and believability of crowd simulations underpins computational
studies of human collective behaviour, with implications for urban design,
policing, security and many other areas. Realism concerns the closeness of the
fit between a simulation and observed data, and believability concerns the
human perception of plausibility. In this paper, we ask two questions, via a
so-called "Turing Test" for crowds: (1) Can human observers distinguish between
real and simulated crowds, and (2) Can human observers identify real crowds
versus simulated crowds? In a study with student volunteers (n=384), we find
convincing evidence that non-specialist individuals are able to reliably
distinguish between real and simulated crowds. A rather more surprising result
is that such individuals are overwhelmingly unable to identify real crowds.
That is, they can tell real from simulated crowds, but are unable to say which
is which. Our main conclusion is that (to the lay-person, at least) realistic
crowds are not believable (and vice versa).
