In a car outside a convenience store in Flint, Michigan, in late 2016, Kemo Parks handed his cousin Dequavion Harris a gun. Things happened quickly after that: Witnesses saw Harris “with his arm up and extended” toward a red truck. Shots rang out. The wounded driver sped off but crashed into a tree. EMTs rushed him to the hospital. He was dead on arrival.
Two weeks later, Harris was arrested following a car chase. He and Parks were charged with the murder of Darnyreouckeonu Jones-Dickerson, known to his friends as Kee-Kee. At trial, a family friend testified that Jones-Dickerson and Harris had been feuding; Harris believed that Jones-Dickerson had been involved in the 2014 murder of his cousin Dominique Fuller. Parks and Harris were both convicted and sentenced to life in prison without the possibility of parole.
Parks’ lawyers appealed the sentencing, arguing that it was unconstitutional. Parks was 18 at the time of his conviction, and his legal team asserted that it constituted “cruel and unusual punishment” to put an 18-year-old away for life. They cited several landmark U.S. Supreme Court cases: 2005’s Roper v. Simmons, which abolished the death penalty for people under 18, and 2012’s Miller v. Alabama, which held that those 17 and younger could not be sentenced to life in prison without the possibility of parole, sometimes referred to as LWOP. The lawyers argued that the age cutoffs established in those cases should be extended upward.
The case eventually made its way to the Michigan Supreme Court, and in one filing, a group of neuroscientists, psychologists, and criminal justice scholars submitted an amicus brief in support. “Drawing the line at 18 for when mandatory LWOP cannot be constitutionally imposed is, from a scientific perspective, both arbitrary and underinclusive,” they wrote. “From a scientific perspective, a person’s 18th birthday is not a rational dividing line for justifying LWOP or similar sentences because the brain continues to develop and change rapidly across all the relevant metrics for several more years.”
The appeal was successful. In July, Michigan Supreme Court judges ruled in Parks’ favor, raising the state’s age cutoff for mandatory life without parole from 18 to 19. But the decision was not unanimous. In her dissent, Justice Elizabeth Clement explained her misgivings about shifting age cutoffs. “I fear that the majority’s opinion is the first step in making it the Court’s ongoing task to reconcile the Legislature’s sentencing scheme with every jot and tittle of new scientific evidence,” she wrote. In their affidavit, scientists had offered evidence that brain development occurs up to at least age 25, and Clement expressed concern that such evidence would spur future changes. “I assume that in the coming years we will hear cases arguing that we should extend Miller’s protection to those in their early twenties as well,” she wrote. “As our understanding of neurological development continues to evolve in the future, must we reevaluate the line between youth and adulthood every few years?”
Clement’s question exemplifies a fundamental tension between science and policy. Science is fluid: Our understanding evolves with new findings, and the work of uncovering the truth is never complete. Scientists dislike the word prove; they prefer to say that the evidence strongly suggests, because who knows what new finding could turn things around? Law, on the other hand, demands rigidity. You can do this but not that. Reconciling these two disparate approaches has always created friction.
But when it comes to neuroscience and punishment, it’s getting worse: A growing body of research strongly suggests that brain development continues well into people’s 20s and beyond. There’s no hard cutoff at which most people have a “mature” brain, and there’s unlikely ever to be one even as scientists conduct more and more studies of how our minds work. There aren’t even clear indicators to test for that would signal that an individual’s brain is now grown and that it’s time to hold them accountable to a higher standard. One of the few things we can say for sure: To draw a hard line at 18 is to get it perilously wrong.
Our legal and social systems struggle with how to account for this uncertainty. Now we’re beginning to confront it head-on when young people make poor or even horrible decisions, and learning whether we can truly embrace an openness to change and fluidity—not only within the law but within ourselves.
Over the past two decades, scientists have amassed new evidence of how our brains develop across our life spans, thanks to the advent of fMRI machines, which allow researchers to peer into subjects’ heads as they think. There is strong scientific consensus that people’s decision-making abilities can evolve between their early and late 20s—but the very idea of a “fully mature” brain is specious because our brains continue changing throughout our lives. In November, I explained how a game of cultural telephone resulted in the widespread but misguided belief that brains are “fully mature” at 25. That age is now frequently held up as a new marker of adulthood, and some people have made dubious claims in the name of neuroscience: that we can’t even truly consent to marriage or sex before that age.
This neuroscience has been cited in arguments for policy change as well. Because young adolescents are still developing, it’s a public health risk to recruit high school students into the military, a public health researcher and science writer argued in a 2011 paper. California Sen. Dianne Feinstein pointed to similar evidence to argue that the minimum age for purchasing semiautomatic weapons should be raised from 18 to 21. “Too young to buy a handgun or alcohol—and too young to buy assault weapons,” she wrote in a Los Angeles Times op-ed in June. That same body of work has also been cited by anti-trans lawmakers to claim that people under age 25 should not have access to gender-affirming medical treatments, or that people under age 18 shouldn’t even be allowed to change their pronouns. “The current evidence does not support the use of puberty blockers, hormone treatments, or surgical procedures for children and adolescents, considering the importance of puberty to brain development, with the pre-frontal cortex (which is responsible for executive functions, such as decision making) continuing to develop until approximately 25 years of age,” an April 2022 press release from the Florida Department of Health claimed.
In some cases, the neuroscience of adolescent decision making should inform law and policy. When judges sentence juveniles who have committed a rash crime, or policymakers determine the conditions under which young people should operate dangerous machinery like cars and guns, neuroscience evidence provides important context about how young people make decisions and the factors that can lead them astray during that process.
But in other cases, advocates merely gesture at “science” to make their argument sound more legitimate. The problem is that unless you’re an expert, it’s not always clear which positions are actually supported by evidence. That contradiction creates a dangerous space that allows people to dismiss all neuroscience evidence, rather than doing the hard work of sorting out what’s true and what isn’t.
That played out in the courts well before Justice Clements voiced her concerns about adjusting law to be consistent with the latest science. Back in 2005, the American Psychological Association submitted an amicus brief in Roper v. Simmons, a case examining the constitutionality of executing juvenile defendants who committed crimes before the age of 18. The APA wrote that people under 18 are “not yet mature in ways that affect their decision-making” and should not be put to death. During oral arguments, Justice Anthony Kennedy pointed out that this stance appeared to contradict an earlier amicus brief, submitted in 1989 for the case Hodgson v. Minnesota, in which gynecologist Jane Hodgson challenged the state of Minnesota’s law that people under 18 must notify their parents before seeking an abortion. In that brief, the APA argued that adolescents are able to make educated decisions about their reproductive rights by midadolescence without the involvement of their parents. “By age 14 most adolescents have developed adult-like intellectual and social capabilities,” the APA wrote. During Roper, Kennedy asked whether the APA had “flip-flopped” on the issue—though he eventually wrote the opinion for the court prohibiting the execution of juvenile defendants. In his dissent, Justice Antonin Scalia mentioned this apparent contradiction: “The APA, which claims in this case that scientific evidence shows persons under 18 lack the ability to take moral responsibility for their decisions, has previously taken precisely the opposite position before this very Court.”
If you really look at the science, though, the two views are not opposed. Kennedy and Scalia assume that young people are either capable of making decisions or not—but that’s not how decision making works. Context and environment play huge roles in any decision: What kind of decision is being made? How much time does a person have to make it? Are there distractions present, or are people pressuring them to make a choice? What’s the emotional weight of the decision, and what other baggage might this person be bringing to the table? Even simple, inconsequential decisions can feel fraught; I’ve been known to lose it when faced with deciding what to cook for dinner after a long day. The nuances of the situation matter to any decision, and research suggests that’s especially true for young people’s decision-making skills under pressure or stress.
Like the APA wrote in its Hodgson amicus brief, young people’s general cognitive skills, including their ability to reason, don’t change much after the age of 14 or so. What does change with age is the ability to reason while distracted; emotions and peer pressure are more likely to hamper decision making in teens and early twentysomethings. Consider how a young person may be an attentive driver during their license exam, when they’re sitting in a quiet car with an adult and fully focused on the road. But once you put that same person behind the wheel with four chatty friends, blaring music, and a barrage of texts at their fingertips, they’re more likely to make bad decisions.
So circumstances matter—and it’s important to note the different circumstances around making a decision about abortion versus committing a crime. Choosing whether to go through with an abortion is generally not something that happens in the heat of the moment; in fact, a majority of states require people seeking abortions to wait at least a day before completing the procedure. (Giving people time and space to carefully consider an abortion is helpful in theory. In reality, however, mandatory waiting periods serve to deter those seeking abortions by complicating the logistics involved, and opening a window through which to push misinformation.) Committing a crime, on the other hand, is often a spur-of-the-moment decision—in the back seat of a car, when Harris asked Parks to hand him a gun, it’s unlikely Parks had more than a few seconds to make the call.
That’s not to say 18-year-olds like Parks aren’t culpable for bad decisions they make. But could that context mean the difference between, say, a 20-year prison sentence and life without the possibility of parole?
Determining how decision-making research should translate into policy is hard work—and not all scientists agree on how to do it. There is consensus among scientists that brain development—and in particular development in brain areas important for decision making—continues into people’s 20s, but every researcher I’ve spoken with about this issue has been clear that there is no magical point at which a brain is “fully developed” or “adult-like,” particularly because no two brains mature at the same rate. Yet the law requires drawing a bright line.
Even in the Parks case, the experts who submitted the affidavit didn’t all agree on what age cutoff to argue for. They agreed an 18-year-old like Parks should not be sentenced to life in prison without parole, but what about a 20-year-old? A 24-year-old?
Arielle Baskin-Sommers, a psychology professor at Yale who was among the researchers who signed the affidavit, told me that in her estimation, 25 feels like a reasonable cutoff, with enough research to back it up: There are studies showing clear differences between how people both older and younger than 25 make decisions. But she acknowledges that 25 is not a hard stop, given the huge amount of variation in individual development, and it’s unclear whether it’s the “right” age. “Science hasn’t told us where that perfect line is,” she said.
Other scientists are hesitant to argue for a cutoff at 25. “I’m very comfortable saying that we should raise the Roper protection age from 18 to 21, but beyond that, I’m not sure,” said Larry Steinberg, a psychologist at Temple University who was not a part of the group that wrote the Parks affidavit. Part of his stance is based on science—it’s a more conservative read on the current body of work. But, Steinberg said, it’s also practical; 21 is more likely to fly with judges and the general public. At 21, you’ve already earned some trappings of adulthood: You can vote, join the military, provide informed consent. It’s a milestone age associated with gaining even more adult privileges, like buying alcohol and gambling. “Arguing to raise the age of death penalty eligibility to 21 doesn’t make people go crazy,” Steinberg said. “If you start to say, ‘Well, for the purposes of criminal culpability, people aren’t adults until they’re 29 years old,’ people would look at you like you’re out of your mind. But when you say 21, people say, ‘Yeah, that seems like a good age.’ ”
In talking with Steinberg and other researchers, I was surprised by the role of strategy in scientists’ legal and policy work. But in truth, ignoring public opinion would be foolhardy; any successful translation of neuroscience into policy must consider whether society is ready to adopt these policies. BJ Casey, a psychologist at Yale, told me that her colleagues have asked why she’d argue for raising the cutoff to only 21 when the scientific evidence suggests that the brain continues to develop long beyond that. “The reason why we’re doing that is because in this country, it wasn’t that long ago where we would still give a death sentence or life without parole to a 17-year-old,” Casey said. “These are baby steps with our science—it’s where we get the most consensus.”
The Parks group is continuing to work together in filing briefs in other cases that could benefit from neuroscience expertise. They just filed an amicus brief to the Massachusetts Supreme Court, weighing in on two cases in which teenagers were sentenced to life without parole. They plan to keep pushing the envelope toward 25, but with the knowledge that there will likely need to be those “baby steps” before getting there, or even going beyond that. “Realistically, we’re probably going to get less than that—but if you ask for 25, you might get 21,” Baskin-Sommers said. Either way, the group believes that the science they presented leaves the door open for extending age cutoffs beyond 18 or 19.
We know that nothing special is guaranteed to happen at 25, or 21, or 18—yet our legal system dictates hard limits. As we learn more about development over the life span, perhaps it will become prudent to keep raising those cutoffs and to consider other mitigating factors; already, scientists and lawyers have argued that judges take into account other brain-altering conditions during sentencing, like concussions, tumors, and dementia. The law need not respond to every “jot and tittle of new scientific evidence,” as Clements put it, but shift our systems when there’s a substantial body of evidence that shows that people’s decision making might be compromised by injury, illness, or age.
Neuroscience has often been invoked in policy and legal discussions to determine people’s culpability, or their readiness for rights and responsibilities. But perhaps it can serve as a guide for how to change our policies and law to scaffold measured decision making. Based on what we know, instituting a waiting period for gun purchases could deter young people from committing rash violence. Mail-in voting could support better decision making by allowing people to carefully consider issues and candidates, rather than making quick picks in a voting booth. Within the legal system, supporting young people can go a long way in steering them away from rash, bad decisions. Sarah Wakefield, a child and adolescent psychiatrist at Texas Tech, said that the juvenile justice program she works with in West Texas saw greater success when it went beyond just providing services therapy or clinical diagnoses. “We’ve really tried to understand what’s happening with these kids and how to support them,” she said, and in the eight years since she’s started her work there, she’s noticed the effects. “The change in that revolving door of recidivism has been significant.”
It’s too late to know if that support might have been helpful for Kemo Parks, and it’s not yet clear if his sentence will be reduced; the Michigan Supreme Court ruling merely outlaws mandatory life without parole for people younger than 19. But his case, together with the 2022 companion case People v. Poole, which establishes that the cutoff applies retroactively, could lead to many adolescents’ resentencing, and for others to avoid life without parole altogether. After the 2016 ruling in Montgomery v. Louisiana determined that outlawing mandatory LWOP sentences for people younger than 18 applied retroactively, thousands of people received shorter sentences, and hundreds were released.
The Parks case, and others like it, represents a change in the legal system that we’re likely to see for years to come. “I would not be surprised if every single person who is incarcerated in Michigan for a mandatory life-without-parole sentence and was between the ages of 18 to 25 at the time of their conduct has filed or plans to file some sort of motion for future parole eligibility, either in their ongoing direct appeal proceedings or in some sort of collateral motion,” said Matt Nguyen, an attorney at the firm Cooley LLP who represented more than a dozen neuroscience and psychology scholars pro bono in the Parks case. “That is going to really develop the conversation around much-needed rehabilitative services for late adolescents.” As we continue to grapple with how to apply neuroscience to law and policy, that conversation is one we need to keep having, even if it’s messy. Science can help us better understand how young people reason and make decisions, but it can’t tell us which laws or policies to enact, or what justice looks like. That hard work is up to us.