The answer surely ought to be no. But getting to that answer isn't as straightforward as it might seem. The central constitutional question relates to the Fifth Amendment, which states that "no person ... shall be compelled in any criminal case to be a witness against himself." In interpreting the Fifth Amendment, courts have distinguished between testimonial evidence, which is protected from compelled self-incriminating disclosure, and physical evidence, which is not. A suspected bank robber cannot refuse to participate in a lineup or provide fingerprints. But he or she can decline to answer a detective who asks, "Did you rob the bank last week?"

So is the information in a brain scan physical or testimonial? In some respects, it's a mix of both. As Dov Fox wrote in a 2009 law review article, "Brain imaging is difficult to classify because it promises distinctly testimonial-like information about the content of a person's mind that is packaged in demonstrably physical-like form, either as blood flows in the case of fMRI, or as brainwaves in the case of EEG." Fox goes on to conclude that the compelled use of brain imaging techniques would "deprive individuals of control over their thoughts" and be a violation of the Fifth Amendment.

But there is an alternative view as well, under which the Fifth Amendment protects only testimonial communication, leaving the unexpressed thoughts in a suspect's head potentially open to government discovery, technology permitting. In a recent law review article titled "A Modest Defense of Mind Reading," Kiel Brennan-Marquez writes that "at least some mind-reading devices almost certainly would not" elicit "communicative acts" by the suspect, "making their use permissible under the Fifth Amendment." Brennan-Marquez acknowledges that compelled mind-reading would raise privacy concerns, but argues that those should be addressed by the Fourth Amendment, which prohibits unreasonable searches and seizures.

That doesn't seem right. It would make little sense to provide constitutional protection to a suspected bank robber's refusal to answer a detective's question if the thoughts preceding the refusal--e.g., "since I'm guilty, I'd better not answer this question"--are left unprotected. Stated another way, the right to remain silent would be meaningless if not accompanied by protection for the thinking required to exercise it.

And if that weren't enough, concluding that compelled brain scans don't violate the Fifth Amendment would raise another problem as well: In a future that might include mature mind-reading technology, it would leave the Fourth Amendment as the last barrier protecting our thoughts from unwanted discovery. That, in turn, would raise the possibility that the government could get a search warrant for our thoughts. It's a chilling prospect, and one that we should hope never comes to pass.

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