The Myth of the Double- Edged Sword: An Empirical Study of Neuroscience Evidence in Criminal Cases

INTRODUCTIONA little explanation can go a long way . . . the difference between life and death.1In 2010, shortly after escaping from prison, John McCluskey killed a retired couple in order to steal their camping trailer.2 The crime was horrific: McCluskey and two accomplices shot the defenseless couple inside their trailer and then set their truck ablaze with their bodies inside.3 Yet a jury rejected the death penalty, instead sentencing McCluskey to life in prison without the possibility of parole.4 The jury's life-over-death choice was seemingly influenced by the defense's introduction of brain scans indicating substantial damage to McCluskey's frontal lobe.5 According to legal analysts, the jury viewed McCluskey's brain abnormalities as a mitigating factor that decreased his level of culpability and ability to plan or intend such a crime, rather than as an aggravating factor that heightened his future danger to society.6Courtroom battles over mitigating and aggravating evidence are a common aspect of capital cases, but the unprecedented use of neuroscience evidence in these battles has led to some striking outcomes.7 In 2010, for example, a judge ruled brain mapping evidence admissible for the first time, noting its "ability to provide vital information on brain injury and impairment."8 The defendant, Grady Nelson, was convicted in Miami of first degree murder after stabbing his wife sixty times, and then also stabbing his step-children.9 Despite Nelson's appalling crimes, the jury declined a death sentence,10 with some jurors noting in post-verdict interviews that neuroscience evidence of Nelson's mental incapacity dissuaded them from issuing a death sentence.11The rising acceptance of neuroscience evidence has fueled heated debate regarding its impact on the criminal justice system.12 The criminal law has focused on the human mind and mental states since the seventeenth century, yet the field of neuroscience is relatively young.13 The first use of the term neuroscience did not even occur until 1963.14 The term is defined in varying ways, but the definition provided by the American Association for the Advancement of Science is representative: neuroscience is "the branch of life sciences that studies the brain and nervous systems [including] . . . brain processes such as sensation, perception, learning, memory, and movement."15 Recent neuroscience research focuses on an even newer discipline-that of cognitive neuroscience, which combines cognitive science, psychology, and neuroscience to examine the mechanisms of the mind, such as motor function, language, higher cognitive functions, emotions, and consciousness.16Key criminal law concepts of culpability depend on the internal workings of individuals' minds. Revelations about a defendant's level of intentionality or consciousness are just some examples of areas where new discoveries could improve the criminal justice system.17 Yet, neuroscience evidence can be portrayed as a potential "double-edged sword: it may diminish [a defendant's] blameworthiness for his crime even as it indicates that there is a probability that he will be dangerous in the future."18 This Article refers to this misconception as the "myth of the double-edged sword." Specifically, much of the debate surrounding the intersection of neuroscience and the criminal law centers on the mistaken assumption that neuroscience evidence will abdicate violent criminals of all responsibility for their crimes- especially those like McCluskey and Nelson.19 In contrast, others fear that such evidence could bolster predictions of defendants' purported future danger to society, thereby unfairly generating longer prison terms or even the death penalty.20 Meanwhile, media accounts of some particularly controversial cases have alarmed the public with inaccurate narratives of how courts use neuroscience evidence and how neuroscience fits into the framework of the criminal justice system.21 The complexity of these legal issues will only expand as the science progresses and becomes increasingly com22 mon in courtrooms. …