Imagine you’re a juror in a murder trial. The defense attorney wheels in a large monitor displaying colorful brain scans of the defendant. An expert witness points to areas highlighted in blue and red, explaining that these images show abnormalities consistent with schizophrenia. The attorney argues that these scans prove the defendant couldn’t tell right from wrong when he committed the crime. As you look at the images, you can’t help but notice how scientific, authoritative, and compelling they appear. After all, you’re looking directly at their brain.

But will you or any other juror make the “right” call when faced with information they may not fully understand?

Actus Reus and Mens Rea

Before diving into brain scans, it’s important to understand two fundamental principles that typically determine criminal liability in the United States: actus reus and mens rea.

Actus reus (Latin for “guilty act”) refers to the physical act of committing a crime. This is usually the more straightforward element to prove, because it is typically based on objective action, such as pulling the trigger, taking property, or striking the victim.[1]

Mens rea (Latin for “guilty mind”) refers to the mental state or intent behind the act. This is the harder element to prove. It often looks at considerations such as whether the defendant intended to act or whether it was an unfortunate accident, and whether they knew or should have known their conduct was wrong.[2] Different crimes require different levels of intent. Murder typically requires intent to kill, while manslaughter might involve recklessness rather than specific intent.[3]

Both elements must typically be present for someone to be found guilty of a crime. The state can’t convict someone of first-degree murder simply because they caused a death; the prosecution must also adequately prove they had the requisite mental state.

Attorneys have started using neuroimaging as evidence to argue that abnormal brain scans demonstrate that the killer lacked the mental capacity to form this necessary intent, or that they couldn’t distinguish right from wrong due to mental illness. In essence, lawyers are trying to use neuroscience to prove their client lacked mens rea for a given criminal charge.[4]

How Courts Decide Whether to Admit Brain Scans as Evidence

When a party wants to introduce scientific or technical evidence like brain scans, courts don’t simply accept it at face value. After all, jurors likely don’t have the expertise needed to make credibility determinations when it comes to neuroimaging. In 1993, the U.S. Supreme Court decided the criteria for expert testimony in a case called Daubert v Merrell Dow Pharmaceuticals, Inc.. The Daubert Court noted that experts must testify to scientific knowledge that will assist a jury to better understand the facts.[5] If so, the scientific evidence must be reliable, as shown through testing, peer review and publication, potential rate of error, and general acceptance in the relevant scientific community.[6]

The Daubert court also referred to Federal Rule of Evidence 702 and 403.[7] Federal Rule of Evidence 702 requires that expert testimony be helpful to the jury, based on sufficient facts and data, properly tested with scientific methods, and appropriately applied to the case at hand.[8] Federal Rule of Evidence 403 allows evidence to be thrown out if it might mislead the jury, among other criteria such as wasting the jury’s time.[9]

In 2012, the Sixth Circuit used the Daubert criteria in a case called United States v. Semrau. In Semrau, the defendant was a doctor who was charged with healthcare fraud and attempted to introduce functional Magnetic Resonance Image (fMRI) test results showing he was “generally truthful” when claiming he tried to follow proper billing practices in good faith.[10] An fMRI is a technique for measuring changes in blood oxygenation and flow in the brain, which occurs in response to neural activity.[11] However, the signal is nonspecific, since it’s an average of millions of cells.[12] This means that it cannot easily differentiate between specific lobes of the brain, nor can it tell us exactly what a person is thinking.

Ultimately, the Semrau court decided to exclude this evidence due to reliability problems. One of the reasons was because the expert witness noted that fMRI lie detection had “a huge false positive problem,” where truth-tellers were incorrectly identified as liars 60-70% of the time.[13] A study in 2009 asked participants to commit a “mock crime” or stealing and damaging CDs, and reported that fMRI may have high sensitivity, but low specificity.[14] The study noted that this result meant an fMRI test may be helpful to ‘‘rule out’’ an innocent suspect, but not very helpful in ‘‘ruling in’’ a guilty suspect.[15]

Brain Scans to Prove Lack of Criminal Responsibility

In Commonwealth v. Chism, decided in 2025 by the Massachusetts Supreme Judicial Court, the defense of a 14-year-old brought in a structural MRI (sMRI) brain scan containing detailed images of his brain’s anatomy.[16] The scans show volumetric abnormalities (differences in the size of certain brain structures) consistent with schizophrenia. When the defendant committed first-degree murder, aggravated rape, and armed robbery, his lawyer used brain scan evidence to argue that the 14-year-old couldn’t understand right from wrong due to mental illness.[17]

The court relied on a 2014 multidisciplinary consensus report from Emory University, which concluded that “the practice of performing imaging studies on a defendant in order to shed light on brain function or state of mind at the time of a prior criminal act is problematic.”[18] The key reason is because a brain scans taken months or years after a crime occurred cannot tell us what was happening in the defendant’s brain at the moment they committed the criminal act.[19] The court also noted methodological issues because the control group (the “normal” brains the defendant’s scans were compared against) wasn’t age-matched to the 14-year-old defendant, making the comparison scientifically questionable.[20]

What This Means for the Future

Does this mean neuroimaging evidence will never be admissible in criminal cases? Not necessarily. Courts seem to have left open the possibility that as science advances and gains broader acceptance, such evidence might meet admissibility standards in the future.

However, the timing issue remains. Criminal law asks whether a defendant had a particular mental state at a specific moment in the past, while neuroimaging shows us what a brain looks like now or how it responds to stimuli in a current testing situation. Bridging that temporal gap requires scientific advances that don’t yet exist.

As neuroscience continues to advance, courts will likely continue to grapple with how and whether brain imaging should influence criminal responsibility. As legal scholar Francis Shen notes, the goal is not to wait for “magical tools,” but to adopt an entrepreneurial “What now?” mentality.[21] Perhaps the way forward is to define clear expert witness guidelines for what type of neuroimaging can be used in the courtroom, or to create better jury instructions that lead to a rightfully skeptical jury.

Understanding these issues will affect how we balance scientific advancement with legal protections, how we determine criminal responsibility, and ultimately, how we define what it means to have a “guilty mind” in an age where we can peer inside the brain itself.

References

[1] Uri Maoz & Gideon Yaffe, What Does Recent Neuroscience Tell Us About Criminal Responsibility?, 3 J.L. & Biosciences 120, 122 (2015).

[2] Id. at 122–23.

[3] Id. at 130.

[4] Neal Feigenson, Brain Imaging and Courtroom Evidence: On the Admissibility and Persuasiveness of fMRI, 2 Int’l J. L. Context 233, 234 (2006).

[5] Daubert v. Merrell Dow Pharms., Inc., 509 U.S. 579, 588 (1993).

[6] Id. at 593–95.

[7] Daubert, 509 U.S. 594-95.

[8] Fed. R. Evid. 702.

[9] Fed. R. Evid. 403.

[10] United States v. Semrau, 693 F.3d 510, 515 (2012).

[11] Nikos K. Logothetis, What We Can Do and What We Cannot Do With fMRI, 453 Nature 869, 869 (2008).

[12] Id. at 876.

[13] Semrau, 693 F.3d 518.

[14] F. Andrew Kozel et al., Functional MRI Detection of Deception After Committing a Mock Sabotage Crime, 54 J. Forensic Sci. 220, 228 (2009).

[15] Id.

[16] Commonwealth v. Chism, 495 Mass. 358, 360, 370–71 (2025).

[17] Id.

[18] Id. at 376–77.

[19] Id. at 376.

[20] Id.

[21] Francis X. Shen, Law and Neuroscience 2.0, 48 Ariz. St. L.J. 1043, 1085 (2016).