Psychophysical studies on confidence construction are often grounded in bidimensional signal detection theory (SDT) and its relatives. However, these studies often stand on oversimplified assumptions of (1) bidimensional variance-equality and (2) bidimensional statistical independence. The present study simulated two-alternative forced-choice and confidence rating performances, incorporating more empirically plausible variance-covariance structures. One prominent observation is that superior metacognitive accuracy can be achieved when one applies a heuristic in which the response-incongruent dimension of information is ignored. This is because such heuristic takes advantage of the specific unequal-variance structure, which paradoxically cannot be easily exploited if both dimensions are evaluated together. Furthermore, under a variety of internal statistical structures, this simple heuristic predicts dissociations of objective decision and subjective metacognition, which have been empirically observed. Also, it provides a tentative account of some behavioral features of blindsight. Therefore, this surprisingly simple decision heuristic may inspire novel perspectives on metacognition and consciousness.