Twenty years ago, the Supreme Court ruled in Roper v. Simmons that sentencing individuals under 18 to death was unconstitutional under the Eighth and Fourteenth Amendments.
The Court cited “scientific and sociological studies” that found adolescents lacked maturity, were more “impetuous and ill-considered” in their decision-making, and were highly susceptible to peer influence. These factors made them “less culpable” than adults, and therefore, the Court reasoned, they should not face the death penalty.
A recent article by the Death Penalty Information Center highlights the 30-year record, including extensive research since Roper that now suggests that the same developmental challenges affecting juveniles also apply to emerging adults—those aged 18, 19, and 20. As one study found, “changes in impulsivity and sensation-seeking are present through emerging adulthood, which could explain differences in risk-taking behaviors between this age group and adults.” These findings have led some experts to question whether the justice system should continue to impose extreme punishments, such as life without parole or the death penalty, on young adults whose cognitive abilities are still developing.
One of the key studies supporting this argument was conducted in 2011 by Dr. Paige Harden and Elliot Tucker-Drob, who examined the trajectory of impulsivity and sensation-seeking from adolescence through young adulthood. Their study of 7,640 individuals between the ages of 12 and 24 found that “impulsivity declined with age until leveling off in the mid-20s,” aligning with neuroscience research showing that the prefrontal cortex—the brain region responsible for impulse control and rational decision-making—continues developing into a person’s mid-20s.
In contrast, sensation-seeking behavior was found to “increase until peaking around age 16, thereafter very slowly declining through emerging adulthood and beyond to the mid-20s.” The study suggested that this trend is tied to the brain’s socioemotional system, which governs emotions, novelty-seeking, and reward sensitivity. The research supports what scientists call a “dual-system model” of adolescent and young adult development, in which the brain’s emotional systems mature earlier than its cognitive control systems, leading to a prolonged period of heightened risk-taking.
These findings challenge the notion that an 18-year-old is fully capable of making mature, rational decisions. Instead, the evidence suggests that “emerging adults, like adolescents, are more likely to engage in risk-taking behavior when surrounded by peers” and struggle with impulse control in emotionally charged situations.
A 2016 study by Dr. Alexandra Cohen and colleagues further demonstrated how emerging adults struggle with cognitive control under emotional pressure. The study compared decision-making abilities across three age groups: 13- to 17-year-olds, 18- to 21-year-olds, and 22- to 25-year-olds. The researchers found that “relative to adults, both teens and emerging adults demonstrated diminished cognitive performance when presented with negative cues (i.e., images of fearful faces).” However, when not under emotional duress, emerging adults performed similarly to older adults.
This study suggests that while emerging adults may appear capable of rational thinking in calm, controlled settings, their decision-making abilities break down in high-pressure, emotionally charged situations. This has major implications for criminal justice policy. As the researchers noted, “in emotional situations, that is, situations that require quick-decision-making, or ‘hot cognition,’ emerging adults may perform poorly when compared to adults.” Given that many crimes occur in high-stress, emotionally intense situations, this research raises serious questions about the fairness of holding emerging adults to the same standard of culpability as fully mature adults.
Another critical factor in evaluating the culpability of young adults is peer influence. A 2005 study by Dr. Margo Gardner and Laurence Steinberg found that both adolescents and emerging adults take significantly greater risks when in the presence of their peers. The study examined risk-taking behaviors among three groups: adolescents (13-16), emerging adults (18-22), and adults (24 and older). The researchers found that while all groups engaged in more risk-taking when among peers, “middle and late adolescents (from 13 to 22 years old) were more susceptible to this peer influence in comparison with adults 24 and older.”
This susceptibility to peer pressure has played a decisive role in criminal cases, particularly in cases involving group crimes. In one well-documented case, five young people in Texas were sentenced to death for the group rape and murder of two teenage girls as part of a gang initiation. Three of them—Peter Cantu, Jose Medellin, and Sean Derrick O’Brien—were 18 at the time of the crime and were executed. The two remaining members of the group, Raul Villareal and Efrain Perez, had their sentences commuted because they were 17 at the time and therefore covered by Roper.
This case highlights the arbitrary nature of treating 18-year-olds as fully culpable adults while granting legal leniency to 17-year-olds, despite evidence showing that their developmental characteristics are nearly identical.
Since Roper, the Supreme Court has continued to recognize that young people are fundamentally different from adults in ways that affect their culpability. In Graham v. Florida (2010), the Court ruled that sentencing juveniles to life without parole for non-homicide offenses was unconstitutional. Two years later, in Miller v. Alabama (2012), the Court extended that reasoning, ruling that mandatory life without parole sentences for juveniles were unconstitutional.
Yet, despite mounting scientific evidence, the legal system continues to treat 18-, 19-, and 20-year-olds as fully mature adults. This approach ignores what we now know about brain development and decision-making in emerging adults. Some jurisdictions have begun to rethink this. In Illinois, lawmakers have proposed raising the age of juvenile jurisdiction to include 18- and 19-year-olds, allowing them to be tried in juvenile rather than adult court. In Germany and the Netherlands, juvenile sentencing frameworks have long been extended to young adults up to age 21.
Thirty years after Roper v. Simmons, the research on emerging adulthood is clear: the cognitive and emotional development of 18- to 20-year-olds is still incomplete, making them less culpable than fully mature adults. As one study put it, “the cognitive control of emerging adults is vulnerable to negative emotional influences and marked by continued development of the prefrontal circuitry.” If we accept that juveniles under 18 are not fully responsible for their actions, then the same logic should apply to emerging adults who exhibit similar psychological and neurological characteristics.
If the justice system is to align with modern scientific understanding, legal protections should be extended to young adults in the same way they have been for juveniles. Expanding sentencing protections would not mean excusing criminal behavior, but rather ensuring that punishments are fair and proportionate to the individual’s level of culpability. Just as Roper once recognized that youthfulness should be a mitigating factor in sentencing, it is time to recognize that emerging adulthood is a distinct developmental stage—one that the justice system can no longer afford to ignore.
