The real key here is to look at who does the selecting. The danger comes from the self selected, if you have a crowd of random people and you choose one at ran

The real key here is to look at who does the selecting. The danger comes from the self selected, if you have a crowd of random people and you choose one at random, then the chances that they have malicious intent are related to the incidence of that in the population; OTOH if you ask for volunteers, or worse, if someone spontaneously volunteers then the chance is way higher...