STATISTICAL DECISION THEORY WAS USED AS A MODEL OF HUMAN DECISION MAKING. 1 OF 2 DICE, D1, WITH 3 BS AND 3 OS ON ITS 6 FACES, OR D2, WITH 4 BS AND 2 OS, WAS SELECTED AND THROWN 5 TIMES. SS GUESSED WHICH DIE HAD BEEN SELECTED, ON THE BASIS OF THE RESULTS OF THE 5 THROWS. 120 TRIALS WERE GIVEN WITH AN UNBIASED PAYOFF MATRIX, FOLLOWED BY 60 TRIALS WITH EACH OF 4 BIASED PAYOFF MATRICES. 3 GROUPS OF 8 SS EACH WERE USED, THE GROUPS BEING GIVEN DIFFERING AMOUNT OF INFORMATION RELEVANT TO THE DECISION TASK. WITH THE UNBIASED PAYOFFS, SS' RESPONSES WERE MORE THAN 90% OPTIMAL, EXCEPT WHEN A NORMATIVE MODEL WOULD PREDICT CONFUSION. BIASING THE PAYOFFS DID AFFECT RESPONSES, BUT NOT TO THE EXTENT PREDICTED BY THE MODEL. DIFFERENTIAL INSTRUCTIONS APPARENTLY HAD NO EFFECT UPON THE OPTIMALITY OF BEHAVIOR. SOME POSSIBLE AMENDMENTS TO A NORMATIVE MODEL OF BEHAVIOR WERE SUGGESTED TO ACCOUNT FOR THE OBSERVED BEHAVIOR. (PsycINFO Database Record (c) 2006 APA, all rights reserved)