Key Takeaways
- The Supreme Court has not addressed whether the government may compel a brain-based memory test, so the constitutional status of cognitive liberty remains unsettled.
- The Fifth Amendment protects only testimonial evidence, and because Schmerber v. California held that a compelled blood draw is not testimonial, it is unclear where brain data would fall.
- Current fMRI technology needs a still, cooperative subject, so an unwilling person can defeat it with small movements, which makes forced scanning impractical for now.
If technology could look inside your brain and read what you remember, could the government force you to undergo the scan? Would the Constitution protect your unspoken thoughts?
In this TalksOnLaw Brief, Joel Cohen talks with Emily Murphy, Professor of Law and Harry & Lillian Hastings Research Chair at UC Law San Francisco (formerly UC Hastings), whose research focuses on the intersection of neuroscience, behavioral science, and law. She explains the emerging idea of cognitive liberty, how the Fourth and Fifth Amendments might apply, and why current technology is far from ready for forced use.
Cognitive Liberty and the Fourth Amendment
Professor Murphy describes a set of questions now gathered under the heading of cognitive liberty. One is a Fourth Amendment question: does the right of the people to be secure in their persons, houses, papers, and effects extend to a right to keep our unexpressed thoughts private? If the government could, in effect, open the curtains and look deep into your brain, would that be an unreasonable search?
Are Your Thoughts Testimonial?
The issue also presses on the Fifth Amendment privilege against self-incrimination, which protects testimonial evidence. Professor Murphy asks whether unexpressed or automatic thoughts, detected by brain imaging with no outward testimonial sign, count as testimony. Her answer is that we do not really know. She points to Schmerber v. California, where the Supreme Court held that blood alcohol content is not testimonial, so a compelled blood draw did not violate the privilege, even though that evidence can be the slam-dunk case for a DUI. Thoughts seem much closer to what we say than to blood alcohol, but she notes that the line between bodily integrity and cognitive liberty has not been drawn. Public safety could push the question, as in hypotheticals about terrorism or a ticking bomb, where the government might claim an overwhelming interest in the mental privacy of someone who refuses to talk.
The Technology Is Not There Yet
Professor Murphy explains that the most powerful method, functional magnetic resonance imaging (fMRI), requires extensive equipment, cannot be used on anyone with ferrous metal in their body, and requires the subject to hold very still in a loud, clanging machine for a long time. Results can be distorted by slight head movements, by swallowing, or simply by thinking about something else. fMRI therefore needs a highly compliant subject, and a hostile witness could defeat it with little more than a twitch of the head, at least with the technology that currently exists.
What to Know Now
The constitutional questions Professor Murphy raises remain open. The Supreme Court has not addressed compelled brain imaging, and the governing framework still comes from cases such as Schmerber and Doe v. United States, which describes testimonial evidence as communication that discloses the contents of a person's mind. Scholars disagree about whether brain data would be treated more like a blood sample or more like compelled testimony. Meanwhile, much of the policy attention has shifted to consumer neurotechnology, such as headsets and wearables that record brain activity. Colorado and California in 2024, and Montana and Connecticut in 2025, added protections for neural data to their privacy laws, and a federal bill, the MIND Act, was introduced in the Senate in 2025 to study the issue. Those laws regulate companies rather than police, but they reflect growing concern for mental privacy. For now, the practical limits Professor Murphy describes keep compelled memory detection a question for future courts rather than today's investigations.
