Reaction time-related compression underlies overestimation of time-constrained reachability

Humans routinely estimate whether an action can be completed within a limited time window, yet the mechanisms underlying such implicit evaluations are not fully understood. Across three experiments (N = 52 in total), we used a psychophysical paradigm to quantify reachability judgments in an arm-reaching task. Participants consistently judged that they could complete the reach even when the available duration was substantially shorter than their actual execution time, indicating an overestimation of time-constrained reachability. This bias contrasts with explicit motor imagery, where imagined durations typically exceed actual execution times. Crucially, by dissociating reaction time (RT) and movement time (MT) components, we show that this overestimation likely arises from selective temporal compression of RT-related component, while MT-related component is largely preserved. Misperception of external duration did not account for the effect. These findings reveal a previously unrecognized cognitive bias and provide new insights into how humans prospectively evaluate their own motor capabilities.