AuthorAvery, Joseph J.

    The scientific study of the structure and function of the nervous system and brain, what today is called neuroscience, is an ancient undertaking. (1) Yet, over the past few decades, advances in neuroscientific technology likely have been more significant than those from the previous millennia combined. As a result, the field has waxed in salience and in impact. (2) Or rather, it has grown in potential impact, as scientists are just beginning to unpack the findings these new technologies are yielding and to apply them to various fields, including medicine (3) and law. (4)

    It is on this latter nexus that this article focuses: how is neuroscientific evidence impinging or threatening to impinge the U.S. legal system, and how are courts addressing this? The answer to this question can be construed as dyadic. On the one hand, neuroscientific evidence is, quite plainly, scientific and technical evidence; accordingly, it is subject to the same vetting to which all such evidence is subject, with attention paid to relevancy, reliability, validity, false positives, and the standards promulgated in Daubert, (5) Frye, (6) and Federal Rules of Evidence 403 and 702, among others. Neuroscientific evidence also involves machine testimony, which raises difficult legal questions and of which jurisprudence remains unsettled. (7) On the other hand, recent neuroscience unsettles seemingly settled philosophical issues undergirding the foundation of much U.S. law, including notions of free will, but also including matters such as what it means to self-incriminate, and whether a line can be drawn between physical and mental suffering.

    Both of these aspects of neuroscience's impact inform judicial behavior, raising philosophical--almost existential--concerns as well as more quotidian, banal ones. In this article, I focus on but two areas in which there have been developments in the use of neuroscientific evidence. The first is the so-called "brain defense," a term that typically refers to post-conviction appeals premised on diminished cognitive function, (8) but which I expand to include a related class of legal arguments, including, inter alia, those relating to human development. The second area is the testimonial one, in which I focus on neuroscience's impact on pain and suffering conceptions and determinations, memory, and deception detection.

    My discussion of these two broad areas yields an overview of the state of neuroscience in U.S. law. The discussion is but prefatory, though, to highlighting how U.S. courts are struggling to accommodate neuroscience. Here, I focus on three areas among many in which the use is inconsistent: forms of abnormal brain functioning; bodily versus mental harms; and credibility determinations. I show that jurisprudence relating to neuroscientific evidence is largely unsettled, sometimes conflicting, and seems likely to face increasingly pressing legal questions.

    I next discuss future directions, as neuroscience is poised to impact myriad issues in the law that I do not discuss in the preceding sections. Many of these points of impact relate to potential further improvements in neuroscience, including the eventual arrival of advanced neural interfaces.

    Lastly, the final section picks up a thread that runs through the succeeding sections: while courts should be able to resolve a number of current inconsistencies in the use of neuroscientific evidence, neuroscience is changing fundamental legal principles and putting ever increasing pressure on courts as gatekeepers.


    Neuroscience is a small field to the extent that its focus is limited to the central nervous system.9 It is a large field, however, in that the brain impacts most every important feature of human life. Thus, any discussion of neuroscience and law threatens to expand almost infinitely. In this paper, I narrow the scope by discussing only two --albeit significant--areas: brain defenses and testimonial matters.

    While these two areas will necessitate discussion of evidence at the sentencing stage, where there is a low bar for admissibility,10 it is worth grounding the discussion with a brief overview of admissibility at the liability/guilt stage. Here, the admissibility of neuroscientific evidence is governed by the rules that apply to scientific evidence generally. (11) Federal Rule of Evidence 403, which allows for the exclusion of relevant evidence "if its probative value is substantially outweighed by a danger of... unfair prejudice, confusing the issues, misleading the jury," or generally being superfluous, may apply. (12) The task before courts is bifold. First, they must consider the inferential chain that leads from the brain scan to the relevant legal issue and ask whether there are too many suspect links. Second, they must consider the jurors and whether they are capable of assessing this chain, albeit with the aid of expert witnesses, for themselves.

    Neuroscientific evidence is garnered via use of a number of machines, including electroencephalography ("EEG"), electrocorticography ("ECoG"), and functional magnetic resonance imaging ("fMRI"), although still other machines are used. (13) These machines are not all created equally. EEG has greater temporal resolution than fMRI but worse spatial resolution. (14) Within fMRI technology alone, there are differences among machines, with some providing greater resolution than others. (15) ECoG is superior to both EEG and fMRI, producing, so to say, clearer pictures with fewer time gaps--perhaps a thousand captures per second--but is highly invasive, requiring partial skull removal and placement of electrodes on the brain itself. (16) For these reasons, ECoG is only used with those who have preexisting conditions, such as persistent seizures, that require brain surgery. (17)

    The evidence received via these machines is evaluated by a number of labs in universities across the country. (18) As always in science and research, experimental rigor and the reliability of results varies from lab to lab, study to study. Moreover, the results of the studies typically are highly technical and involve statistical probabilities of correlation and, sometimes, causation. Thus, such evidence is accompanied to the witness box by expert witnesses. So, we must also think of Federal Rule of Evidence 702, application of which is guided by a trilogy of U.S. Supreme Court cases: Daubert v. Merrell Dow Pharmaceuticals; General Electric v. Joiner; (19) Kumho Tire v. Carmichael. 20 Rule 702 allows an expert witness to testify if: (1) the expert's knowledge will help the trier of fact to comprehend the evidence; (2) "the testimony is based upon sufficient facts or data;" (3) "the testimony is the product of reliable... methods;" and (4) the witness has applied the methods reliably "to the facts of the case." (20)

    1. Brain Defenses

      The typical brain defense is the kind that emerges post-conviction in atypical cases. (22) For example, shortly after Eric Williams was sentenced to death for a triple homicide, his attorney filed a motion in which he argued that newly discovered fMRI evidence showed that Mr. Williams' brain had been damaged by prior head trauma and that this damage prevented Mr. Williams from normative regulation of his emotions and behavior. (23) Mr. Williams' brain was "broken." (24)

      In this article, I expand the term "brain defense" to include other instances in which neuroscientific evidence is proffered in order to buttress arguments that, on account of the defendant's brain, he or she should not be treated as a typical defendant. Thus expanded, brain defenses include issues of adolescence, abnormal brain functioning, illness/injury/trauma to the brain, addiction, and insanity.

      1. Adolescence

        Adolescents take risks. (25) They smoke, have unprotected sex, shoplift --and they do so at much higher rates than do those who are eighteen and older. (26) Such claims are anecdotally true, evident to most of us who have spent time with adolescents. They also are statistically backed, (27) and they have led to such measures as age restrictions on when an individual is allowed to drive on public roads. (28)

        Neuroscience provides a different type of evidence in support of these same claims. It provides evidence of the physical matter of the brain--that it changes in time, especially during the adolescent period, and that its physical operations are closely related to and help to explain manifestations of risky behavior. (29) Most major universities that house departments of neuroscience have labs that focus solely on human development. (30)

        Developmental neuroscience is beginning to offer legally relevant insights, (31) a claim borne out by the fact that such evidence has made its way to the Supreme Court. In 2005, the Court ruled on Roper v. Simmons, (32) a case involving the commission of murder by a seventeen-year-old who was subsequently sentenced to death after turning eighteen. (33) Justice Kennedy, writing for the Court, addressed whether the Eighth and Fourteenth Amendments to the U.S. Constitution prohibit the execution of a juvenile offender. (34) In concluding that such punishment is prohibited, Justice Kennedy cited psychology and neuroscience literature. (35) Indeed, the case was extensively briefed by the American Psychological Association, which made use of conclusions drawn from MRI studies. (36)

        This is an apt place for my discussion to begin, as neuroscience was, in fact, cited by both the majority and the dissent. (37) Justice Scalia, joined by Chief Justice Rehnquist and Justice Thomas, wrote in dissent:

        We need not look far to find studies contradicting the Court's conclusions. As petitioner points out, the American Psychological Association (APA), which claims in this case that scientific evidence shows persons under 18 lack the ability to take moral responsibility for their decisions, has previously taken precisely the opposite...

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