This study experimentally examined people’s decision making in the Newcomb’s problem. We observed that Savage’s sure-thing principle and Kolmogorovian law of probability was violated. Also, the degree of the violation increased as the reward size increased. By adopting quantum decision theory, we further quantified interference effect as a quantum phase factor. The quantum phase also depended on the reward size; i.e., it increased as the reward size in the unknown box in the Newcomb’s problem increased. Future directions in the application of the present theory to studies in quantum decision theory and neuroeconomics are discussed

uncertainty, Newcomb’s paradox, quantum decision theory, risk