Most police searches today are authorized by citizens' consent, rather than probable cause or reasonable suspicion. The main constitutional limitation on so-called "consent searches" is the voluntariness test: whether a reasonable person would have felt free to refuse the officer's request to conduct the search. We investigate whether this legal inquiry is subject to a systematic bias whereby uninvolved decision-makers overstate the voluntariness of consent and underestimate the psychological pressure individuals feel to comply. We find evidence for a robust bias extending to requests, tasks, and populations that have not been examined previously. Across three pre-registered experiments, we approached participants ("Experiencers") with intrusive search requests and measured their behavioral compliance and self-reported feelings of psychological freedom. Another group of participants ("Forecasters") reported whether they would comply if hypothetically placed in the same situation. Study 1 investigated participants' willingness to allow experimenters access to their unlocked personal smartphones in order to read through the search histories on their web browsers-a private sphere where many individuals feel they have something to hide. Results revealed that whereas 27% of Forecasters reported they would permit such a search, 92% of Experiencers complied when asked. Study 2 replicated this underestimation-of-compliance effect when individuals were asked to permit a search of their purses, backpacks, and other bags-traditional searches not eligible for the heightened legal protection extended to digital devices. Study 3 replicated the gap between Forecasters' projections and Experiencers' behavior in a more representative sample, and found it persists even when participants' predictions are incentivized monetarily.