What makes a juvenile? One of the brightest lines in the US legal system is that of the age at which one can be tried and sentenced as an adult. What is so special about 18? The age has a special cultural significance (ironically, perhaps stemming from the presence of a legal cutoff at that point), but is there any neuroscience that actually supports the use this point for the line, or any line at all?

Jay Aronson of Carnegie Mellon University provides an excellent review of the topic (Aronson, 2009). One of the most important instantiations of the idea of age-related culpability is the verdict of Roper v. Simmons, a case in which the court held that those under age of 18 could not be sentenced to death on the grounds that they were not fully culpable for their actions. This case cites Thompson v. Oklahoma, “their irresponsible conduct is not as morally reprehensible as that of an adult.” In addition it was supposed that “The reality that juveniles still struggle to define their identity means it is less supportable to conclude that even a heinous crime committed by a juvenile is evidence of irretrievably depraved character.” Analysis of the Brief of Respondent for this case indicates that the Justices actually did consider some neurological and cognitive evidence in defining these things. The evidence, and court’s the interpretation of it, was as follows:

MRI evidence indicates that the frontal lobes are not fully developed in 16-17 year olds. The unfinished areas, specifically the prefrontal cortex, are strongly involved in for emotional inhibition and impulse control, decision making and planning, and decisions involving reward and risk. The particular way in which these PFC areas are underdeveloped is a lack of myelination and a lack of synaptic ‘pruning’ (essentially learning). Thus, 16-17 year olds have an underdeveloped “seat of intentionality.” Finally, white matter (which is indicative of well myelinated neurons and appropriately selective connectivity) tends to keep increasing into the early twenties (much of this evidence was cited as coming from Goldberg et. al., 2001).

Slightly more recent literature has corroborated the supposition the PFC white matter correlates with the quality of higher cognitive function. Schmithorst, et. al., 2005 used diffusion tensor imaging to correlate measures of fractional anisotropy (which itself measures the amount of white matter in this context) with measures of IQ. However, it should be noted that there is of course some amount of individual difference in the rate/amount of myelination present at a given age (Sowell et. al., 1999).

What is truly fascinating about the use of this neurological evidence is the fact that, in the words of Santha Sonenberg, “Thus, even without his serious cognitive deficits, at nineteen, his brain was not that of an adult.” ‘His,’ referring to the juvenile defendant in United States v. Kurt W., (D.C. Super. Ct. 2005), but generalizing to anyone under the age of 18. The defense in Roper v. Simmons wanted to “have both anatomical and cognitive normalcy and pathology defined by age rather than by some diagnosable medical condition or mental state,” and this is indeed what happened. However, instead of leading to standard evaluations of individuals’ prefrontal development and something like a continuum of intentionality (and perhaps culpability), Roper v. Simmons led to a blanket precedent that 18 was to be the cutoff for full culpability in cases of the death penalty. While the neurological evidence does not make this seem an unreasonable place to draw a line, the fact that individuals differ in PFC development must call into question the practice of drawing a universal line in the first place.

Citations

Jay D. Aronson, Neuroscience and Juvenile Justice, 42 AKRON L. REV. 917 (2009).

Goldberg, Elkhonon, The Executive Brain: Frontal Lobes and the Civilized Mind (2001)

Memorandum in Aid of Sentencing, United States v. Kurt W., at 7 (D.C. Super. Ct. 2005)

United States v. Kurt W., (D.C. Super. Ct. 2005) (Juvenile Criminal Case Under Seal).

Elizabeth R. Sowell et al., In Vivo Evidence for Post-Adolescent Brain Maturation in Frontal and Striatal Regions, 2 Nature Neuroscience  859 (1999)

Schmithorst VJ, Wilke M, Dardzinski BJ, Holland SK.  Cognitive functions correlate with white matter architecture in a normal pediatric population: a diffusion tensor MRI study. Hum Brain Mapp. 2005 Oct;26(2):139-47.

Roper v. Simmons, 543 U.S. 551 (2005)