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

Humans continuously estimate whether an action can be completed within a limited time window, yet the mechanisms underlying such implicit evaluations remain unclear. Across three experiments (N = 52 in total), we used a novel psychophysical paradigm to quantify reachability judgments in an arm-reaching task. Participants consistently judged actions as possible even when the available time was 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 execution times. Crucially, by dissociating reaction and movement components, we show that this overestimation arises from selective temporal compression of the premotor phase, while movement time 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.