The irresponsible conduct of juveniles is not as morally reprehensible as that of an adult.

The Wisconsin Juvenile Court Intake Association, the professional organization that represents Wisconsin juvenile court intake workers, has explained that there is “[a] wealth of recent research in adolescent brain development indicates that most adolescents simply do not act like adults. They lack the capacity to consistently make behavioral choices based on long-term planning and reasoning. Neuropsychological research shows that important aspects of brain maturation remain incomplete until well after age 21.” Wisconsin Juvenile Court Intake Association, Statement of Philosophy One key example is that “[w]hen adolescents make choices involving risk, they do not engage the higher-thinking, decision-and-reward areas of the brain as much as adults do. This can lead adolescents to actually overstate rewards without fully evaluating the long-term consequences or risks involved in a situation.” Neir Eshel et al., “Neural Substrates of Choice Selection in Adults and Adolescents,” Neuropsychologia 45, no. 6 (2007): 1270–1279. This is because the limbic system, which helps to process and manage emotion, is developing during adolescence so that adolescents experience more mood swings and impulsive behavior than adults. Rebecca L. McNamee, “An Overview of the Science of Brain Development,” (slide presentation, dated May 5, 2006), at http://www.juvjustice.org/sites/default/files/resource-files/resource_108.pdf. Researchers therefore posit that the courts may wish to consider the notion that individuals are “less guilty by reason of adolescence.” MacArthur Foundation Research Network on Adolescent Development and Juvenile Justice, “Issue Brief 3: Less Guilty by Reason of Adolescence,” Philadelphia, PA, 2006, http://www.adjj.org/downloads/6093issue_brief_3.pdf. A 2013 report from the National Research Council, “Reforming Juvenile Justice: A Developmental Approach,”http://www.nap.edu/catalog/14685/reforming-juvenile-justice-a-developmental-approach, suggests that legal responses to juvenile crime should take into consideration that an “imbalance in developing brains” is linked to a poorer ability to self-regulate and make decisions. See also, Gately, “Experts: Brain Development Should Play Bigger Role in Determining Treatment of Juvenile Offenders”Juvenile Justice Information Exchange (2013), http://jjie.org/experts-brain-development-should-play-bigger-role-in-determining-treatment-of-juvenile-offenders/105927/Courts now recognize this science. For instance, in Roper v. Simmons, 543 U.S. 551; 125 S. Ct. 1183, 1190 (2005), the United States Supreme Court noted that “we have established the propriety and affirmed the necessity of referring to the ‘evolving standards of decency that mark the progress of a maturing society’ to determine which punishments are so disproportionate as to be cruel and unusual.” Id. citing Trop v. Dallas, 356 U.S. 86, 100-101, 78 S. Ct. 590 (1958). The Constitution does not permit a court to dispose of a juvenile case by a categorical approach without consideration of a juvenile’s chronological age and the hallmark features of being a juvenile— immaturity, impetuosity, and failure to appreciate risks and consequences –as well as the juvenile’s family and home environment that surrounds him. Miller v. Alabama, 567 U.S. 460, slip at 15 (2012)
Three significant differences which impact juveniles’ culpability were identified in Roper v. Simmons. First, “[a] lack of maturity and an underdeveloped sense of responsibility are found in youth more often than in adults and are more understandable among the young. These qualities often result in impetuous and ill-considered actions and decisions.” Id. at 1195. “The susceptibility of juveniles to immature and irresponsive behavior means their irresponsible conduct is not as morally reprehensible as that of an adult.” Id; citing Thompson v. Oklahoma, 487 U.S. 815, 815, 108 S. Ct. 2687 (1998).
Second, juveniles are more susceptible to outside influence and peer pressure than adults.  Id.  “Their own vulnerability and comparative lack of control over their immediate surroundings mean juveniles have a greater claim than adults to be forgiven for failing to escape negative influences in their whole environment.” Id; See, Stanford v. Kentucky, 492 U.S. 361, 395 (1989) (Brennan J. dissenting)
The third significant difference that impacts culpability is that the character of juveniles is not yet well-formed and, therefore, juvenile personality traits are more transitory than those of adults. Id. “From a moral standpoint it would be misguided to equate the failings of a minor with those of an adult, for a greater possibility exists that a minor’s character deficiencies will be reformed…For most teens, [risky or antisocial] behaviors are fleeting; they cease with maturity as individual identity becomes settled. Only a relatively small proportion of adolescents who experiment in risky or illegal activities develop entrenched patterns of problem behavior that persist into adulthood.” Id. at 1196; citing Steinberg & Scott “Less Guilty by Reason of Adolescence: Developmental Immaturity, Diminished Responsibility, and the Juvenile Death Penalty, 58 Am. Psychologist, 1009, 1014 (2003). Finally, in Graham v. Florida, 560 U.S. 48 (2010) it was found that because juvenile’s have diminished culpability there exists a greater prospect for reform.
The scientific studies detailed in the amici briefs submitted in Roper (and relied on by the Court) explain that the differences in juvenile and adult culpability result from the anatomically underdeveloped nature of the juvenile brain. See Brief of Amici Curiae, American Psychological Association, et. al. (“APA Brief”), https://www.apa.org/about/offices/ogc/amicus/roper.pdf and Brief of Amici Curiae, American Medical Association et al. (“AMA Brief”), http://fairsentencingofyouth.org/wp-content/uploads/2014/04/Roper-v.-Simmons-Amicus-Brief-American-Medical-Association-et.-al..pdf. These fundamental differences between juveniles and adults are rooted in very real biological differences between juvenile and adult brains. Modern brain research technology, specifically high resolution magnetic resonance imaging (MRI), has allowed scientists to see that juveniles’ immature behavior is based on “deficiencies in the way adolescents think” that result from the “anatomical immaturity of their brains.” AMA Brief at 6, 10. These studies have revealed that “regions of the adolescent brain do not reach a fully mature state until after the age of 18,” and that “psychological maturity is incomplete until age 19.” Id.
More particularly, these studies have led to two important observations that have enabled scientists to understand juveniles’ behavior and its anatomical basis.  Id. at 11. “First, adolescents rely for certain tasks, more than adults, on the amygdale, the area of the brain associated with primitive impulses of aggression, anger and fear. Adults, on the other hand, tend to process similar information through the frontal cortex, a cerebral area associated with impulse control and good judgment.” Id. The frontal lobes of the brain, which are tied to “a variety of cognitive abilities including decision making, risk assessment, ability to judge future consequences, evaluating reward and punishment, behavioral inhibition, impulse control, deception, responses to positive and negative feedback and making moral judgments,” exert control over the amygdale, which is associated with aggressive and impulsive behavior. Id. at 13-15. However, a still maturing frontal lobe in juveniles is less able to exert control over the amygdale (associated with impulsive and aggressive behavior) than does a fully mature frontal lobe in adults. Id. at 14-15. As teenagers age into adults, the focus of the brain activity shifts from the amygdale to the frontal lobes. Id. at 15.
The second scientific observation that affects our modern understanding of juvenile brain development is the discovery that “the regions of the brain associated with impulse control, risk assessment and moral reasoning develop last.” Id. at 11; see also APA Brief at 10-12. Brain imaging studies have confirmed that the frontal lobes of the brain are structurally immature well into late adolescence and beyond:
The new brain imaging data, supported by data gathered through the older autopsy technique, provides credible evidence that the frontal lobes, which are still developing into adolescence and beyond, are among the last portions of the brain to mature. In other words, the region of the brain associated with impulse control, risk assessment and moral reasoning is the last to form, and is not complete until late adolescence or beyond.” Id. at 18. Emphasis added.
This has led researchers to conclude that regions of the brain are not fully developed until at least age 18, and “(a)dolescents as a group, even at the age of 16 or 17, are more impulsive than adults. They underestimate risks and overvalue short-term benefits. They are more susceptible to stress, more emotionally volatile, and less capable of controlling their emotions than adults.”  Id. at 2.
Relying on the submitted scientific research, the Roper Court concluded that the differences between juveniles and adults “render suspect any conclusion that a juvenile falls among the worst offenders.”  Roper, 125 S. Ct at 1195. The Court explained:
The susceptibility of juveniles to immature and irresponsible behavior means ‘their irresponsible conduct is not morally reprehensible as that of an adult.’ Their own vulnerability and comparative lack of control over their immediate surroundings mean juveniles have a greater claim than adults to be forgiven for failing to escape negative influences in their whole environment. The reality that juveniles still struggle to define their own identity means it is less supportable to conclude that even a heinous crime committed by a juvenile is evidence of irretrievably depraved character. From a moral standpoint it would be misguided to equate the failings of a minor with those of an adult, for a greater possibility exists that a minor’s character deficiencies will be reformed…once the diminished culpability of juveniles is recognized, it is evident that the penalogical justifications for the death penalty apply to them with lesser force than to adults. Id. at 1195-96. Emphasis added.