The Myth Of The 25 Year Old Brain

The Myth of the 25-Year-Old Brain
by Jane C. Hu
excerpted from this article: https://slate.com/technology/2022/11/brain-development-25-year-old-mature-myth.html

"A powerful idea about human development stormed pop culture and changed how we see one another. It’s mostly bunk.

Alexandra Cohen, a neuroscientist at Emory University, said the scientific consensus is that brain development continues into people’s 20s.
But, she wrote in an email, “I don’t think there’s anything magical about the age of 25.”

Yet we’ve seen that many people do believe something special happens at 25. That’s the result of pop culture telephone: As people reference the takeaways from Cohen and other researchers’ work, the nuance gets lost. For example, to add an air of credibility to its DiCaprio theory, YourTango excerpts a passage from a 2012 New York Times op-ed written by the psychologist Larry Steinberg: “Significant changes in brain anatomy and activity are still taking place during young adulthood, especially in prefrontal regions that are important for planning ahead, anticipating the future consequences of one’s decisions, controlling impulses, and comparing risk and reward,” he wrote.

Steinberg is a giant in the field of adolescent development, well known for his four decades of research on adolescent and young adults. The passage YourTango quoted accurately describes the science, but it’s definitely a stretch to imply that it explains Leonardo DiCaprio’s dating history. When we spoke, I told Steinberg his work had been referenced in this way. “Oh no,” he said, laughing. I then asked whether he had insights about where the figure 25 came from, and he said roughly the same thing as Cohen: There’s consensus among neuroscientists that brain development continues into the 20s, but there’s far from any consensus about any specific age that defines the boundary between adolescence and adulthood. “I honestly don’t know why people picked 25,” he said. “It’s a nice-sounding number? It’s divisible by five?”

Kate Mills, a developmental neuroscientist at the University of Oregon, was equally puzzled. “This is funny to me—I don’t know why 25,” Mills said. “We’re still not there with research to really say the brain is mature at 25, because we still don’t have a good indication of what maturity even looks like.”
Maturity is a slippery concept, especially in neuroscience. A banana can be ripe or not, but there’s no single metric to examine to determine a brain’s maturity. In many studies, though, neuroscientists define maturity as the point at which changes in the brain level off. This is the metric researchers considered in determining that the prefrontal cortex continues developing into people’s mid-20s.

That means that for some people, changes in the prefrontal cortex really might plateau around 25—but not for everyone. And the prefrontal cortex is just one area of the brain; researchers homed in on it because it’s a major player in coordinating “higher thought,” but other parts of the brain are also required for a behavior as complex as decision making. The temporal lobe helps process others’ speech and language so you can understand what’s going on, while the occipital lobe allows you to watch for social cues. According to a 2016 Neuron paper by Harvard psychologist Leah Somerville, the structure of these and other brain areas changes at different rates throughout our life span, growing and shrinking; in fact, structural changes in the brain continue far past people’s 20s. “One especially large study showed that for several brain regions, structural growth curves had not plateaued even by the age of 30, the oldest age in their sample,” she wrote. “Other work focused on structural brain measures through adulthood show progressive volumetric changes from ages 15–90 that never ‘level off’ and instead changed constantly throughout the adult phase of life.”

To complicate things further, there’s a huge amount of variability between individual brains. Just as you might stop growing taller at 23, or 17—or, if you’re like me, 12—the age that corresponds with brain plateaus can differ greatly from person to person. In one study, participants ranged from 7 to 30 years old, and researchers tried to predict each person’s “brain age” by mapping the connections in each person’s brain. Their age predictions accounted for about 55 percent of the variance among the participants, but far from all of it. “Some 8-year-old brains exhibited a greater ‘maturation index’ than some 25 year old brains,” Somerville wrote in her Neuron review. Some of those differences might be random genetic variation, but people’s behavior and lived experience contribute as well. “Childhood experiences, epigenetics, substance use, genetics related to anxiety, psychosis, and ADHD—all that affects brain development as well,” said Sarah Mallard Wakefield, a forensic psychiatrist.

All this means that people’s brains can look very different from one another at 25. If we’re leaving it up to neuroscience to define maturity, the answer is clear as mud. The concept of adulthood has been around much longer than neuroscience has been able to weigh in on it. Ultimately, we are the ones who must define the shift from adolescence to adulthood.

But the takeaways from neuroscience are rarely ironclad, which complicates the question of what role these studies should have in shaping policy around the rights and responsibilities of young people. Contrary to what Waxman and many others might believe, neuroscience can be just as squishy as psychology, a field some snobs argue isn’t even a science. Just like psychologists, neuroscientists must make judgment calls about how to collect and interpret data, and there are no right answers for how best to do that. Studying people is messy. “Despite being popularly viewed as revealing the ‘objective truth,’ neuroimaging techniques involve an element of subjectivity,” three health researchers who study adolescents wrote in a 2009 paper.

The choices researchers make in their methodology and data analysis affect their results. Even the participants researchers study biases their data set. (It’s well established that most research is biased toward people in “WEIRD”—Western, educated, industrialized, rich, and democratic—countries.) Plus, the researchers said, “the cognitive or behavioral implications of a given brain image or pattern of activation are not necessarily straightforward.”

In other words, researchers might be able to take a picture or video of the brain, but it’s not always clear what this really shows. The interpretation of neuroimaging is the most difficult and contentious part; in a 2020 study, 70 different research teams analyzed the same data set and came away with wildly different conclusions. Now that tens of thousands of fMRI studies have been published, researchers are identifying flaws in common neuroscience methods and questioning the reliability of their measures.

That’s not to say we should disregard the neuroscience—we just need to acknowledge its limitations. “We are giving neuroscience a starring role where it should have a supporting role,” Steinberg said.

Even with a flimsy basis at best, the real-world consequences of the “brains are fully mature at 25” myth are only beginning to emerge. Some of those are relatively harmless; using this half-truth to explain Leonardo DiCaprio’s dating habits primarily hurts DiCaprio, who hardly needs our sympathy.

In other cases, it could cost lives; anti-trans activists cite this as evidence that young people should not be allowed to access lifesaving, gender-affirming care. The ultimate trajectory of this growing belief—and the profound effect it could have on young lives—is impossible to know, but it’s clear that neuroscience has and will be deployed to shape policy."