Middle Temple Young Barristers' Association

Intellectual Disability as a Defence to the Death Penalty – but is it?

Laura Broome

In 2002, the United States Supreme Court overruled its previous longstanding decision[1] and declared in Atkins v Virginia[2] that the Eighth Amendment of the US Constitution exempts defendants from execution if they meet the criteria for ‘mental retardation’ (now intellectual disability). By reaching such conclusion, the Court explicitly acknowledged that those with intellectual disability, by definition, have diminished capabilities to process and understand information, to employ logical reasoning and to control their impulses.[3]

Although the Court emphasised the importance of such a categorical exception, it did leave the highly important matter of defining the disorder, in addition to any directions on assessment, to the individual states. The Court stated that so long as states generally conformed to the clinical definitions set out in the American Association of Mental Retardation (now the American Association on Intellectual and Development Disabilities) and the American Psychiatric Association in its Diagnostic and Statistical Manual of Mental Disorders, that state measures would be ‘constitutional’ and ‘appropriate’.[4] Both clinical definitions identify three broad criteria: significantly subaverage intellectual functioning and significant limitations in adaptive functioning, both originating before the age of 18.

Having undertaken my internship in Texas, I was able to gain a good understanding of this topic and see first-hand the impact this decision has had on intellectually disabled defendants facing capital charges. As such, this brief article will submit that as a consequence of omitting to establish a strict criterion, the Court has in fact created opportunity for ambiguous interpretations and as a consequence, has permitted state deviations to facilitate what Atkins does not – the imposition of death sentences on intellectually disabled defendants.



i. Intellectual Functioning

Significantly subaverage intellectual functioning is often measured by an individual’s performance on a standardised IQ test.[5] Whilst there has been some discussion surrounding the effectiveness of IQ tests in capital cases generally, an important debate arises over the interpretation of these IQ scores.

The first main issue with the interpretation of such scores relates to the standard error of measurement (SEM). This is defined as ‘a statistical fact reflecting the test’s inherent imprecision and acknowledging that an individual score is best understood as a range.’[6] Namely, five points either side of the IQ score. The lack of a set criterion surrounding the interpretation of IQ scores has allowed the exclusion of intellectual disability claims for some defendants who clearly fall within that category.

In the case of Cherry v Florida[7], an IQ score of 72 rendered the defendant exempt from an Atkins claim, as Florida statute required a score of 70 or less in order to meet the intellectual disability criteria. Even after arguing that the SEM should be taken into consideration, as this would mean the IQ score would fall somewhere between 67 – 77, the court imposed a strict cut-off at 70 and dismissed the intellectual disability claim altogether, without any consideration of the defendants adaptive functioning, contrary to what was held in Atkins. On appeal in 2014[8], the Supreme Court did eventually decide that trial courts must consider the SEM and that Florida’s threshold cap was unconstitutional, widening the scope for an Atkins claim for intellectually disabled defendants. However, the court also held that when a defendant’s score falls within this margin of error, the defendant is required to present additional evidence of their intellectual disability. Thus, although the scope is widened in one sense, the defendant is encumbered with a further obligation of having to present additional evidence, arguably making it much more difficult for defendants in this instance.

Another debate, which I spent a significant amount of time researching in Houston, is the Flynn effect. The Flynn effect recognises ‘the gradual increase of IQ scores on a particular instrument over time.’[9] It has been determined that in the United States, IQ scores rise by 3 points every decade, or 0.33 points annually, since the test was normed.[10] Hence, to counteract the Flynn effect, IQ tests are systematically re-normed.

Whilst undertaking my research, it quickly became clear that courts’ conflicting views of the Flynn effect has also led to inconsistencies within the law. I found that some experts have testified that the Flynn effect is an ‘unexamined scientific concept’[11], which only applies to groups and not individuals[12]. On the contrary, other cases have held that the Flynn effect should be taken into consideration.[13] Nevertheless, failure to account for the Flynn effect can lead to potentially invalid IQ scores that fall outside of the intellectual disability range; ultimately contributing to the execution of defendants who may otherwise have been exempt from the death penalty.[14]


ii. Adaptive Functioning

In order to satisfy the second criterion of significant limitations in adaptive functioning, ‘a person’s ability to deal with the demands of the everyday world must be impaired.’[15] This includes deficits in practical, conceptual and social skills.[16] These deficits are measured using a combination of numerous tools, [17] including standardised tests, clinical evaluations, the reviewing of records, and reports from acquaintances of the defendant.

Although this in itself provides for extensive debate[18], a more specific issue relates to the lack of norming of such methods to the criminal population, as standardised and structured measures of adaptive functioning and behaviours were not intended for prisoners.[19] Although it has been argued that the norming of assessment methods to inmates would be irrelevant and would only assess whether the defendant is more or less ‘adapted’ than his incarcerated peers,[20] it has also been contended that more research in this area should develop norms for the prison population.[21] The use of tests that have not been normed to prisoners is a clear deviation of any standardised test instructions and thus, it should at the very least call into question the validity of such tests in capital cases.

Additionally, measuring adaptive functioning in the case of a defendant means that any tests relating to family and friends are conducted retrospectively. Asking a friend or family member of the defendant to complete a standardised test based on memories from potentially years previous further questions the validity of such methods, and this was an issue that often arose during my internship. There are also additional issues with the credibility of reports from acquaintances of the defendant; asking other people in prison to undertake such tests may give way to bias. Other prisoners or prison guards may be motivated to exaggerate certain things in the hope that doing so will either prevent or support a diagnosis of intellectual disability.

During my time in Houston, it became apparent that using the aforementioned methods to assess adaptive functioning therefore pose significant problems for defendants in capital cases. Such tests could be the determining factor in an intellectual disability diagnosis and if these tests are invalid, the defendant may not receive the correct diagnosis, consequently allowing for the execution of defendants who are in fact intellectually disabled.


iii. Age of Onset

The final criterion for establishing intellectual disability is that the defendant must have met the first two criteria before the age of 18. The burden of proof is this instance is on the defendant, to prove that he or she had in fact met the first two functional limitations prior to this age.

However, this poses significant problems too. As I observed first hand, often and even in instances where there may have been signs of intellectual disability, defendants have not undertaken any standardised tests before turning 18. This could be for many reasons, from a lack of school or family funding to instances where a school may have overlooked such signs in order to avoid any extra responsibility.[22]

The case of State v Stallings[23] demonstrates the problems with this criterion. In this case, although the court accepted that the defendant did meet the first two criteria, no expert for the defence could conclude with certainty that these deficits occurred before the defendant had turned 18 and thus, the defendants Atkins claim was rejected. Although questions were raised in a federal habeas review as to whether or not this imposed an impossible burden on a defendant who was not tested prior to age 18, the court affirmed, stating that this criterion was not unreasonable.[24] Thus, a defendant who the court believed had both subaverage intellectual functioning and significant limitations in adaptive functioning was still not exempt from the death penalty.


iv. Conclusion

Notably, this article has only briefly explored some of the issues surrounding intellectual disability as a defence to the death penalty, based on my research and experience as an intern, and as such, there are many additional issues which could be explored. Nonetheless, it was clear from my time in Texas and it is clear even from these brief issues that although the Atkins exemption has been described as a move towards a more ‘balanced and humane institution’[25], there is still a long way to go before Atkins is applied uniformly across the United States of America. The current state deviations from the clinical definitions create great inconsistency within the law, which ultimately means that some defendants are being held to a higher standard than others when it comes to meeting the criteria for this defence. This risks a haphazard application, and although the current system does exempt some defendants from the death penalty, it still allows what Atkins sought to preclude - the perpetual imposition of death sentences on some intellectually disabled defendants.





[1] Penry v Lynaugh 492 US 302 (1989)

[2] 536 US 304 (2002)

[3] John McGee and Frank Menolascino, ‘The Evaluation of Defendants with Mental Retardation in the Criminal Justice System’ in Ronald Conley, Ruth Luckasson and George Bouthilet (eds) The Criminal Justice System and Mental Retardation (Paul H Brookes Pub Co 1992)

[4] (n 2)

[5] Anna Hagstrom, ‘Atkins v. Virginia: An Empty Holding Devoid of Justice for the Mentally Retarded’ [2009] 27 Law & Ineq 1

[6] Hall v Florida 572 US (2014) 2

[7] Cherry v State 959 US 702 (2007)

[8] (n 6)

[9] John Blume, Sheri Johnson and Christopher Seeds, ‘Of Atkins and Men: Deviations from Clinical Definitions of Mental Retardation in Death Penalty Cases’ (2009) Cornell Law School Legal Studies Research Paper 122/2009, 9

[10] Robert Schalock et al, Intellectual Disability: Definition, Classification, and Systems of Supports (11th ed, American Association on Intellectual and Developmental Disabilities 2010)

[11] Ex Parte Blue 230 SW US 151 (Tex Crim App 2007) 166

[12] ibid

[13] Walker v True 399 US 315 (4th Cir 2005)

[14] See Green v Johnson US Dist 90644 (2006); Johnston v State, 178 US GA App 219 (1986), (although notably, whilst on death row Johnston died of natural causes)

[15] John Blume, Sheri Johnson and Christopher Seeds, ‘An Empirical Look at Atkins v. Virginia and its application in Capital Cases’ [2009] 76 Tenn L Rev 625, 630

[16] American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders (5th ed, American Psychiatric Publishing 2013); (n 10); for a more detailed explanation, see Richard Bonnie and Katherine Gustafson, ‘The Challenge Of Implementing Atkins v. Virginia: How Legislatures And Courts Can Promote Accurate Assessments And Adjudications Of Mental Retardation in Death Penalty Cases’ [2007] 41 Rich L Rev 811

[17] See ibid

[18] (n 16, Bonnie and Katherine)

[19] This is highlighted in the dissent of Judge Elsa Alcala in Moore v Texas 581 US (2017)

[20] Marc Tasse, ‘Adaptive Behavior Assessment and the Diagnosis of Mental Retardation in Capital Cases’ [2009] 16 Applied Neuropsychology 114

[21] K Stevens and J Price, ‘Adaptive Behavior, Mental Retardation, and the Death Penalty’ [2006] 6(3) Journal of Forensic Psychology Practice 1

[22] Although notably, there are certain reasons which may explain the lack of an earlier diagnosis, see (n 10) 8

[23] State v Stallings 107 NC App US 241 (1992)

[24] Stallings v Bagley 56 US 821 (2008)

[25] Dill Ayres, ‘Capital Punishment and the Intellectually Disabled: Controversies, Constitutionality, and the Supreme Court’ [2012] The Trinity Papers 1/1/2012 65 <> 

Background image by DAVID ILIFF. License: CC-BY-SA 3.0