Police can force you to press your thumb to unlock your phone in California. But in D.C., that same act may violate your constitutional right against self-incrimination. Federal courts seem to be split on whether using your body to unlock your mobile device counts as testimony from your mind or mere physical evidence not subject to constitutional protection.

The Ninth Circuit in United States v. Payne says that using your thumb to unlock a device is like giving a fingerprint at booking, just an immutable characteristic that isn’t protected under the Fifth Amendment. The D.C. Circuit in United States v. Brown says that the act of unlocking your phone with your thumb could confirm that you know how to open the phone, and that you have some control over its contents. The difference between these holdings comes down to context and fuzzy concepts about cognitive labor and mental exertion that neither court adequately defines.

The Fifth Amendment guarantees that no person “shall be compelled in any criminal case to be a witness against himself.” This means the government can’t force you to provide incriminating evidence from your own mind in a criminal case against you. They can take your fingerprints, draw your blood, or photograph you and use all of that as evidence against you, because those are just physical facts about your body rather than answers to questions that reveal what you know.

The distinction in this line of cases has turned on the increasingly blurry line between physical and “testimonial” evidence. Forcing someone to say their password is clearly testimonial because it reveals memorized information. Taking someone’s fingerprint is clearly physical because it’s just a part of your body. But modern technology is expanding the gray area between these categories, such that when you use your fingerprint to unlock a phone, we now must ask whether in doing so you are providing a physical trait or revealing that you know how to access the device.

This conflict over how a suspect’s biometric evidence fits within existing interpretations of the self-incrimination clause of the Fifth Amendment may soon collide with an even newer, and perhaps even more legally complex, technological reality. On September 30, Meta begins shipping a $799 EMG device that can read your intention to move from the neural signals it detects at your wrist.

If we can’t agree on whether using your thumb to open a device requires “thinking,” what happens when the interface to do so is purely neural? Is there such a thing as a “purely physical” neural response?

The Split Over Biometric Data

In November 2021, California Highway Patrol officers grabbed Jeremy Travis Payne’s thumb while  detained in the back of their police cruiser, and pressed it to a cell phone they had retrieved from his car. In April of 2024, the Ninth Circuit in United States v. Payne held that Payne’s thumb being pressed to the device “required no cognitive exertion” on his part, and was therefore not a violation of his Fifth Amendment right against self-incrimination. The court reasoned that using “Payne’s thumb to unlock his phone appears no different from a blood draw or fingerprinting at booking,” requiring the use of only an “immutable physical characteristic.” Since Payne had already identified the phone as his own, the compelled use of his thumb didn’t divulge any new thoughts from his mind.

Nine months later, the D.C. Circuit saw things differently in a case with similar facts, United States v. Brown. In Brown, an FBI agent had ordered a January 6th defendant to unlock his phone with his thumbprint. The D.C. Circuit found this use of a thumbprint was a violation of Brown’s Fifth Amendment right against self-incrimination. The court reasoned that Brown’s “act of unlocking the phone represented the thoughts ‘I know how to open the phone,’ ‘I have control over and access to this phone,’ and ‘the print of this specific finger is the password to this phone.’” In other words, the act itself communicated information from the suspect’s mind.

Both courts applied the same constitutional test, asking whether the act was compelled, incriminating, and testimonial. But they split on what makes an act “testimonial.” Specifically, whether it matters that officers physically used Payne’s thumb versus ordering Brown to use his own, and whether the unlocking itself revealed ownership and control (as in Brown) or merely opened an already-identified device (as in Payne). But what’s particularly interesting is how these courts use  terms like “cognitive exertion,” “mental processes,” and “contents of his mind” as determinative of whether the biometric unlocking was testimonial, seemingly grasping for distinctions that the traditional physical/testimonial framework cannot provide.

In my 2012 Stanford Law Review article Incriminating Thoughts, I proposed a way to tease out the different kinds of mental processes that these courts are attempting to define.  I introduced  a spectrum of evidence spanning from identifying (physical traits), to automatic (involuntary responses), to memorialized (stored knowledge), and finally, uttered mental responses  (communicative).

Using this framework, we can see that Payne involves purely identifying evidence, a physical trait used without any mental participation required. While Brown, by contrast, compels  memorialized evidence that becomes uttered, because the command to unlock forces the suspect to access stored knowledge about control and authentication, then communicate that knowledge through the act itself. The unlocking essentially “speaks” what the suspect knows. But until courts explicitly adopt a clear framework like this one that enables them to draw these kinds of distinctions, they will keep talking past each other as technology evolves.

If an officer holds up your phone to unlock it using Face ID while you’re conscious but passive, is that physical evidence or testimonial? You’re awake, but it doesn’t require your mental participation if you’re just sitting there. Now imagine instead that the officer says “look at your phone.” You have to understand the command, choose to comply, and direct your gaze toward the phone. Does this minimal mental activity cross the physical to testimonial legal divide? Current doctrine doesn’t offer a clear answer.

A more coherent approach to whether something is “testimonial” should examine whether the act itself communicates information from the suspect’s mind. In Brown, compelling the suspect to unlock the phone revealed mental content—he knew how to unlock it, had control over it, and could authenticate access. The act communicated these facts even before reaching the contents of the phone. In Payne, by contrast, officers used his thumb as a physical key to open a phone he’d already admitted was his. The thumb itself revealed nothing from his mind, serving only as a mechanical means of access.

Meta’s new Neural Band makes clear just how unworkable our existing approach to the Fifth Amendment will become. The Neural band “translates the signals created by your muscle activity into commands” by detecting neuromotor signals “even before [movement is] visually perceptible.” This consumer device reads your intention to move—pure thought that exists before any physical action.

Will passive neural signatures, your brain’s unique patterns, count as “physical” evidence like a fingerprint? The answer might depend entirely on context. Using neural signals  to identify you could be non-testimonial, but using them to authenticate device ownership might reveal memorialized information (“this is my device”). Will intentional neural commands count as “testimonial” evidence? They’re thought, in the form of physical evidence. Or most troubling, will involuntary neural responses like P300 waves when your brain recognizes familiar images, count as physical or testimonial evidence? These are memorialized information, your brain revealing what it knows, without your conscious awareness or volition.

The traditional physical/testimonial divide simply can’t address these questions. Under the D.C. Circuit’s reasoning, compelling someone to unlock a device through neural intention would clearly violate the Fifth Amendment—it’s accessing thoughts without any physical intermediary. Even under the Ninth Circuit’s narrower view, officers cannot “apply” someone’s neuromotor signals the way they can physically manipulate a thumb.

The paradox here is striking. As interfaces become more “intuitive,” requiring less conscious effort, they might receive less Fifth Amendment protection under a purely physical framework. The easier it becomes to think your way into your device,  the harder it will become to claim constitutional protection when you do so.

Which is why the hairsplitting between the Ninth and D.C. Circuits isn’t just about thumbs versus passwords. It’s about whether the Fifth Amendment protects the cognitive space between your mind and your devices. A space that technology is rapidly eroding.