The rise and fall of the “warrior gene” defense

Abstract

Since a 1993 study suggested that a single gene might predispose an individual to violence ([ 1 ][1]), criminal defendants have argued that their genes made them commit the crime. Through genome-wide association studies, we now understand that thousands of gene variants each contribute a very small amount to behavioral variation between individuals, and the idea that a single gene could cause violent behavior has been rejected. In February, the so-called “warrior gene” theory as a criminal defense was dealt a blow by the New Mexico Supreme Court in the case of State v. Yepez ([ 2 ][2]), but the ruling did not go far enough. By failing to address the current scientific consensus, the court missed an opportunity to create the much-needed persuasive authority for other courts on this issue. We must improve communication between scientists and the legal system to ensure that courts can access and understand the most current science. The defendant in the recent court case, Anthony Blas Yepez, sought to introduce expert testimony that he had a genetic predisposition to violence that prevented him from being able to form the intent necessary to commit second-degree murder. The trial court excluded the testimony—which linked Yepez's “warrior gene” and childhood maltreatment to his ability to form the intent to kill—as scientifically unproven. On appeal, the appellate court upheld his conviction but found that the trial court had acted wrongly in excluding the testimony ([ 3 ][3]). The New Mexico Supreme Court disagreed. It found that the trial court acted reasonably in excluding the testimony as scientifically unfounded. The New Mexico Supreme Court reached the right conclusion but used the wrong analysis. Their opinion ignored the scientific consensus, reached by 2011 and articulated in 2019 ([ 4 ][4]), which rejects a candidate-gene approach that identifies single genes responsible for variations in human behavior. The court wrongly endorsed the trial court's conclusion that findings from research like the original 1993 study had achieved “general acceptance…in the relevant scientific communities” ([ 2 ][2]). The court's mistake sets a dangerous legal precedent and threatens justice itself. The genetic predisposition defense has been used in other cases with starkly diverging results ([ 5 ][5]). Some defendants have been convicted on lesser charges, whereas others have been punished more harshly on the mistaken belief that the defendant was programmed to kill [e.g., ([ 6 ][6])]. This case could have set the record straight and established a precedent on which other courts could rely. We need better engagement between scientists, lawyers, and judges to help our legal institutions move beyond outdated science that wrongly labels defendants as genetically programmed to offend. 1. [↵][7]1. H. G. Brunner et al ., Science 262, 578 (1993). [OpenUrl][8][Abstract/FREE Full Text][9] 2. [↵][10] State v. Yepez , NO. S-1-SC-37216 C/W NO. S-1-SC-37217, 2021 WL 732668 (N.M. Supreme Court, 25 February 2021) 3. [↵][11] State v. Yepez , 428 P.3d 301 (Ct. App. New Mexico, 24 July 2018) 4. [↵][12]National Institute of Mental Health, “Report of the National Advisory Mental Health Council Workgroup on Genomics: Opportunities and challenges of psychiatric genetics” (2019); [www.nimh.nih.gov/about/advisory-boards-and-groups/namhc/reports/report-of-the-national-advisory-mental-health-council-workgroup-on-genomics.shtml][13]. 5. [↵][14]1. W. Bernet et al ., J. Foren. Sci. 52, 1362 (2007). [OpenUrl][15] 6. [↵][16] Schriro v. Landrigan , 550 US 465 (2007). G.E.R. served on the National Advisory Mental Health Council Workgroup on Genomics. [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-4 [5]: #ref-5 [6]: #ref-6 [7]: #xref-ref-1-1 View reference 1 in text [8]: {openurl}?query=rft.jtitle%253DScience%26rft.stitle%253DScience%26rft.aulast%253DBrunner%26rft.auinit1%253DH.%26rft.volume%253D262%26rft.issue%253D5133%26rft.spage%253D578%26rft.epage%253D580%26rft.atitle%253DAbnormal%2Bbehavior%2Bassociated%2Bwith%2Ba%2Bpoint%2Bmutation%2Bin%2Bthe%2Bstructural%2Bgene%2Bfor%2Bmonoamine%2Boxidase%2BA%26rft_id%253Dinfo%253Adoi%252F10.1126%252Fscience.8211186%26rft_id%253Dinfo%253Apmid%252F8211186%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [9]: /lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6Mzoic2NpIjtzOjU6InJlc2lkIjtzOjEyOiIyNjIvNTEzMy81NzgiO3M6NDoiYXRvbSI7czoyMzoiL3NjaS8zNzEvNjUzNi8xMzIwLmF0b20iO31zOjg6ImZyYWdtZW50IjtzOjA6IiI7fQ== [10]: #xref-ref-2-1 View reference 2 in text [11]: #xref-ref-3-1 View reference 3 in text [12]: #xref-ref-4-1 View reference 4 in text [13]: http://www.nimh.nih.gov/about/advisory-boards-and-groups/namhc/reports/report-of-the-national-advisory-mental-health-council-workgroup-on-genomics.shtml [14]: #xref-ref-5-1 View reference 5 in text [15]: {openurl}?query=rft.jtitle%253DJ.%2BForen.%2BSci.%26rft.volume%253D52%26rft.spage%253D1362%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [16]: #xref-ref-6-1 View reference 6 in text