Automatically adjusting confidence scoring functionality is described for a speech recognition engine. Operation of the speech recognition system is revised so as to change an associated receiver operating characteristic (ROC) curve describing performance of the speech recognition system with respect to rates of false acceptance (FA) versus correct acceptance (CA). Then a confidence scoring functionality related to recognition reliability for a given input utterance is automatically adjusted such that where the ROC curve is better for a given operating point after revising the operation of the speech recognition system, the adjusting reflects a double gain constraint to maintain FA and CA rates at least as good as before revising operation of the speech recognition system.