Hanbei Zhou¹ (), Rui Zhe Goh², Ian Phillips³, Chaz Firestone⁴; ¹Johns Hopkins University

Among the most reliable connections between internal mental processing and external behavior is *response time*, with easier, more accurate, and more confident judgments typically made faster. But which aspects of response time are relevant? Whereas psychophysical studies traditionally focus on the time taken to initiate a response, an underexplored measure is the duration of the response itself—not just the amount of time between stimulus onset and keypress (reaction time), but also how long one holds down the key before releasing it (response duration). Response duration is a ubiquitous and freely available data source, yet almost no studies report or analyze it (Pfister et al., 2023). Here, 3 varied experiments demonstrate that response duration reliably predicts subjective confidence, independent of reaction time. In Experiment 1, subjects detected faces within white noise, with difficulty manipulated by varying face opacity. Subjects responded with a keypress (with both keyUp and keyDown events recorded separately), followed by a confidence judgment. Remarkably, subjects held down the response key longer during trials in which they subsequently reported lower confidence, as if making these face-detection judgments in a tentative fashion. The same pattern held in another visual task (judging the coherence of random-dot motion; Experiment 2), and a cognitive task (classifying American cities as geographically Eastern or Western; Experiment 3). In all cases, response duration accounted for variance in confidence that was not predicted by reaction time. Response duration has distinct advantages as a measure of confidence: It taps confidence at the time of judgment (rather than retrospectively), it can be used when traditional confidence judgments are difficult to elicit (e.g., in animals or infants), and it may be less affected by biases associated with explicit reports. Our results suggest that response duration is a valuable and untapped source of information, raising many avenues for future investigation.