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Striking Gold: When does the brain reach maturity?
A shorter summary of the latter parts of An Empirical Introduction to Youth
Let’s play a game. Let’s imagine that we’re digging for gold, where gold in this case are urban myths. Well, not ordinary myths exactly. Scientific myths. We probably won’t find gold, but it’s useful to look. And I have an idea where to look: the brain. Gold has been found here before.
Consider the idea that we only use 10% of our brains:
Researchers suggest that this popular urban legend has existed since at least the early 1900s. It may have been influenced by people misunderstanding or misinterpreting neurological research. The 10% myth may have emerged from the writings of psychologist and philosopher William James. In his 1908 book, The Energies of Men, he wrote, "We are making use of only a small part of our possible mental and physical resources."
Similarly, we often hear that the brain develops until the age of 25. Maybe the 25 number is a myth. Maybe it isn’t. You never know for sure until you look at the data.
Let’s ask Google.
Normally, this is all you need. But today, we’re on Myth Patrol. It may be a fruitless exercise, but let’s dig a little deeper. We never know if we’ll strike gold.
Bad news. The first source is an interview of a brain scientist, Sandra Aamodt. She’s talking about her book, which I downloaded and read, but it doesn’t mention when the brain reaches maturity. Instead, it mostly focuses on childhood brain development.
The NPR Source
Let’s focus on the interview. Maybe the science isn’t settled, and we’ve found a myth for today … bad news for myth hunters. She says it is:
COX: Is this idea that the brains of 18 year olds aren't fully developed a matter of settled science?
AAMODT: Yes. The car rental companies got to it first, but neuroscientists have caught up and brain scans show clearly that the brain is not fully finished developing until about age 25.
Oh well, it’s a win either way. We either find fun myths on these little excursions, or we see that science is working as intended. A few years ago, I would have stopped here. But today, against the odds, I’m going to press on. Maybe I’m wasting my time, but at any rate, let’s look at the evidence Dr. Aamodt refers to. We might learn something. Or we might spot a myth. I do happen to know quite a bit about the brain, after all, owing to my academic pursuits. But probably not as much as an active experimenter. Still, you never know until you know.
COX: To not be too clinical in the spin that we put on this, what parts of the brain are we talking about and what changes happen between the ages of 18 and, let's say, 25?
AAMODT: So the changes that happen between 18 and 25 are a continuation of the process that starts around puberty, and 18 year olds are about halfway through that process. Their prefrontal cortex is not yet fully developed. That's the part of the brain that helps you to inhibit impulses and to plan and organize your behavior to reach a goal.
And the other part of the brain that is different in adolescence is that the brain's reward system becomes highly active right around the time of puberty and then gradually goes back to an adult level, which it reaches around age 25 and that makes adolescents and young adults more interested in entering uncertain situations to seek out and try to find whether there might be a possibility of gaining something from those situations.
COX: So this is important. Are the physiological changes in the brain, in terms of the development of young people, as significant and impactful as the cultural changes and environmental changes that they go through vis-a-vis peer pressure things of that sort?
AAMODT: Well, actually, one of the side effects of these changes in the reward system is that adolescents and young adults become much more sensitive to peer pressure than they were earlier or will be as adults.
So, for instance, a 20 year old is 50 percent more likely to do something risky if two friends are watching than if he's alone.
So, one thing I am immediately familiar with is the research on sensation seeking, which is the term psychologists use for what Dr. Aamodt refers to when she talks about the reward system being more active. And, interestingly enough, it seems like we might have found a small nugget of silver, maybe not gold, here: one study I know of found that sensation seeking, i.e. the tendency to enter uncertain situations to find reward, declines with age throughout the lifespan. It doesn’t plateau at 25. So you can’t scientifically draw a random line through a monotonic section of a graph. It might be more scientific to reason that the point of maturity for this metric is its maximum; then, with aging, it decreases. Sensation seeking often peaks during pubertal years and was long ago directly correlated with pubertal stages: “Boys and girls with more advanced pubertal development had higher ratings of sensation seeking … Sensation-seeking increases from age 10, peaks between 13-16 years, and then declines” (Forbes & Dahl 2010).
I’m inspired. Let’s keep digging. While Dr. Aamodt seems to be wrong about sensation seeking plateauing at the age of 25, her other claims are probably better. She probably just misspoke. Let’s look at peer pressure. I’m also familiar with the literature here. Dr. Aamodt says youth are more sensitive to peer pressure. However, Gardner & Steinberg (2005) found that, among White participants, there were not significant differences between youth scores (mean 19 years), alone or with a group, and adolescent scores (mean 14 years). Furthermore, among white adults, the presence of peers actually led to increased risk taking to a similar degree as in the younger groups. Only in the Black participants is the pattern of increased group pressure at younger ages present. This might indicate that susceptibility to peer pressure is affected by social or environmental factors that might differ between races or age groups. Even more significant is the fact that effect sizes between racial groups are more significant than between white age groups. For instance, d = .29 between White 14 year olds and White 25+ year olds, while d = .69 between Black 14 year olds and White 14 year olds. In other words, the idea that youth under 25 have yet to reach mature judgment abilities due to innate differences in brain structure is not supported by this peer pressure study (and this study is very well known, and I am almost certain that it’s the one Dr. Aamodt had in mind, for what it’s worth, considering the context. And I do believe that it is a fair representation of the literature as a whole).
Hm. We might be hitting some sort of metal here. Let’s check out the second search result and look at the frontal lobes more generally.
The Rochester Source
It doesn’t matter how smart teens are or how well they scored on the SAT or ACT. Good judgment isn’t something they can excel in, at least not yet.
The rational part of a teen’s brain isn’t fully developed and won’t be until age 25 or so.
In fact, recent research has found that adult and teen brains work differently. Adults think with the prefrontal cortex, the brain’s rational part. This is the part of the brain that responds to situations with good judgment and an awareness of long-term consequences. Teens process information with the amygdala. This is the emotional part.
In teens' brains, the connections between the emotional part of the brain and the decision-making center are still developing—and not always at the same rate. That’s why when teens have overwhelming emotional input, they can’t explain later what they were thinking. They weren’t thinking as much as they were feeling.
This same, word-for-word statement is can be found on multiple college websites, with no mention of sharing, including Stanford, UC San Diego, and Rochester. I wonder if they all agreed to post this same writing? No matter. Let’s check out what the data says. We could very soon be rich, if we really are onto something.
The Rochester source says teens have an undeveloped prefrontal cortex relative to the amygdala, so let’s look at the prefrontal cortex first. The structural development of the prefrontal cortex beyond infancy is well researched; one great paper on this subject is a 2004 study led by Dr. Jay Giedd. The study was longitudinal, following a group of thirteen people from 1994 to 2004 with ages at the end of the study ranging from 4-21 years old.
The study found that “Overall, the total [gray matter] volume was found to increase at earlier ages, followed by sustained loss starting around puberty … Frontal and occipital poles lose GM early, and in the frontal lobe, the GM maturation ultimately involves the dorsolateral prefrontal cortex, which loses GM only at the end of adolescence.”
By “end of adolescence”, Giedd means the end of puberty, i.e. 13 or 14, based on the scatter plots he was nice enough to provide:
Hm. Another study says that gray matter declines in the prefrontal cortex, and in all other parts of the brain, throughout the lifespan. This decline is predictable enough that you can predict someone’s age with a mean average error of 5 to 10 years, based on a measurement of their gray matter volume (Jiang et al. 2020). So again, we just might be onto something here. You can’t really scientifically say that maturity lies at 25, in the middle of that life-long decline. And like sensation seeking, this curve hits its maximum during puberty. It looks like that as the brain matures during childhood, gray matter increases, and afterwards it decreases due to aging until death. Is it just a coincidence that this pattern-matches with sensation-seeking?
Another claim is that white matter accumulates in the frontal lobe during the teen years. But for the frontal lobe is particular, one study reports that “white matter volume increased until age forty-four years for the frontal lobes [which includes the prefrontal cortex] and age forty-seven years for the temporal lobes and then declined” (Bartzokis et al. 2001). So again, 25 would be quite a random line to draw.
Let’s look at a study on the amygdala. The Rochester source explicitly references the connection between the amygdala and the rest of the brain. These connections have been found to be mature by the end of puberty: “Unlike results based on chronological age, which showed significant development during childhood, all the tracts except one cluster (localized in the left IFOF) [this was mature by mid-puberty] demonstrated continued immaturities in midpuberty and only became adultlike by the postpubertal stage. This finding suggests that pubertal changes may be more tightly coupled to white matter maturation than had been previously considered.” (Asato et al. 2010) Puberty stages were determined “by Tanner staging.” In 1970, white boys reached the last Tanner stage, the postpubertal stage, on average at the age of 14.9 years (Marshall & Tanner 1970).
We may have found gold. Maybe. Perhaps it’s just a nugget. But we’re definitely seeing a pattern here with puberty. Hm… it does make sense for the brain to reach its full function at the end of puberty, evolutionarily speaking. It’s when your body is at full strength, and it’s when many animals go off on their own. It seems pretty maladaptive to give people mature, dangerous, reproducing bodies for 10 years without the mental capacity to control themselves and succeed. Interesting.
Behavioral and Psychometric Studies
Even more interesting, I’ve found some behavioral and psychometric studies on judgment and brain function.
Two show that teens are at least equal in driving abilities when compared older people with the same level of experience. Below is a graph from one of them showing that crash rates are predicted by experience, not age:
(Also see Wayne & Miller 2018.)
British driving data backs us up further. Youth are more likely to pass the driving test, possibly indicating greater mental fitness compared to more aged participants.
Also of interest are British drunk-driving figures. We expect teens to be the most immature; in the UK, the drinking age is 18 and so is the driving age. So 18 year olds should drunk-drive the most if the brain matures at 25.
It appears that 25-29 year olds are the most likely age group to drive drunk in the UK. Hm.
One laboratory study in this literature measured the ability of teens and older people to delay a $1000 dollar award by offering them cheaper rewards immediately. A “discount rate” was calculated based on how little money they were willing to go to get the money immediately.
This test is generally considered to be reliable, and once again it shows maturity at the end of puberty.
Another study used a driving game where the risk of crashing was ambiguous and drivers were rewarded for driving through lights. While younger teens exhibited riskier driving behavior, the 16-17 year old age group experienced mature levels of risky driving on par with those of adults (Steinberg et al. 2008).
We’ll end with IQ data. Ryan et al. (2000) found that “scores on Matrix Reasoning are at a peak for persons at 16 to 17 years of age and differences [decline] first become evident for persons 45 to 54 years of age.” So IQ reaches maturity at the end of puberty.
While this review is not quite exhaustive, we may want to bring in some diggers and see if we can’t open a mine here. It’s possible that we found a myth. Maybe the media made a mistake. This deserves further research.
References can be found in the mine the author later dug, aka the book known as An Empirical Introduction to Youth. It was concluded after a laborious analysis that grant money was biased toward portraying teens and young adults as immature for the purpose of promoting universal education, producing the myth that was chipped at above. The book is for sale on Amazon and free on libgen.