This paper examines some sequential effects found in the behavior of observers in the signal‐detection situation. The Tanner‐Swets‐Green model treats the subject as a stable decision maker operating on information from a noisy but unbiased transducer. It is suggested that this model may be profitably replaced by one in which a bias is introduced by a simple finite‐state machine which makes the subjects' behavior a function not only of the present stimuli but also of past stimuli and responses.