The modification or maintenance of behaviour by accidental (also adventitious, incidental, or spurious) relations between responses and reinforcers, as opposed to those either explicitly or implicitly arranged . Classes of superstitions include. simple superstitions, in which responses are maintained, usually unstably, by reinforcers delivered independently of behavior; concurrent superstitions, in which one response is maintained by reinforcers produced by a different response; sensory superstitions, in which identical contingencies maintain different performances during different stimuli; and topographical superstitions, in which reinforcers produce and maintain a response topography that varies over a much narrower range than that specified by the limits of the operant class. Interpretations in terms of superstitious behaviour must be drawn with caution, because it is inevitably variable either within or across organisms, and because performances that superficially appear to be superstitious can sometimes be shown to depend instead on subtle contingencies. Many human superstitions depend on rule-governed behaviour rather than, or in addition to, accidental contingencies (e.g., to be superstitious about breaking mirrors, one need not first have seven years of bad luck after doing so).