1. A method comprising steps of: receiving through a communication network, in a server coupled to the communication network, a request from a user to access a service; in response to the request, presenting to the user a minority set of objects of size N and a majority set of objects of size greater than N, wherein N is a natural number, and wherein the objects in the majority set share at least one common attribute that is expected to be apparent to a human observer, that is absent among objects in the minority set, and that is attributable to each object in the majority set independently of other objects in the majority set; requesting, from the user, selection of at least one object, from among the objects in the minority or majority set, that satisfies at least one predetermined criterion related to the at least one common attribute; receiving, from the user, a selection of at least one object; determining whether each of the at least one user-selected object satisfies the at least one predetermined criterion; and determining whether to allow the user to access the service based on whether each of the at least one user-selected object satisfies the at least one predetermined criterion related to the at least one common attribute.