And The Brain Goes Marching On!

In Tuesday’s newsletter, we asked you “when does the human brain stop developing?” and got the above-depicted, below-described, set of responses:

• About 64% of people said “Never”
• About 16% of people said “25 years”
• About 9% of people said “65 years”
• About 5% of people said “13 years”
• About 3% of people said “18 years”
• About 3% of people said “45 years”

Some thoughts, before we get into the science:

An alternative wording for the original question was “when does the human brain finish developing”; the meaning is the same but the feeling is slightly different:

• “When does the human brain stop developing?” focuses attention on the idea of cessation, and will skew responses to later ages
• When does the human brain finish developing?” focuses on attention on a kind of “is it done yet?” and will skew responses to earlier ages

Ultimately, since we had to chose one word or another, we picked the shortest one, but it would have been interesting if we could have done an A/B test, and asked half one way, and half the other way!

Why we picked those ages

We picked those ages as poll options for reasons people might be drawn to them:

• 13 years: in English-speaking cultures, an important milestone of entering adolescence (note that the concept of a “teenager” is not precisely universal as most languages do not have “-teen” numbers in the same way; the concept of “adolescent” may thus be tied to other milestones)
• 18 years: age of legal majority in N. America and many other places
• 25 years: age popularly believed to be when the brain is finished developing, due to a study that we’ll talk about shortly (we guess that’s why there’s a spike in our results for this, too!)
• 45 years: age where many midlife hormonal changes occur, and many professionals are considered to have peaked in competence and start looking towards retirement
• 65 years: age considered “senior” in much of N. America and many other places, as well as the cut-off and/or starting point for a lot of medical research

Notice, therefore, how a lot of things are coming from places they really shouldn’t. For example, because there are many studies saying “n% of people over 65 get Alzheimer’s” or “n% of people over 65 get age-related cognitive decline”, etc, 65 becomes the age where we start expecting this—because of an arbitrary human choice of where to draw the cut-off for the study enrollment!

Similarly, we may look at common ages of legal majority, or retirement pensions, and assume “well it must be for a good reason”, and dear reader, those reasons are more often economically motivated than they are biologically reasoned.

So, what does the science say?

Our brains are never finished developing: True or False?

True! If we define “finished developing” as “we cease doing neurogenesis and neuroplasticity is no longer in effect”.

Glossary:

• Neurogenesis: the process of creating new brain cells
• Neuroplasticity: the process of the brain adapting to changes by essentially rebuilding itself to suit our perceived current needs

We say “perceived” because sometimes neuroplasticity can do very unhelpful things to us (e.g: psychological trauma, or even just bad habits), but on a biological level, it is always doing its best to serve our overall success as an organism.

For a long time it was thought that we don’t do neurogenesis at all as adults, but this was found to be untrue:

How To Grow New Brain Cells (At Any Age)

Summary of conclusions of the above: we’re all growing new brain cells at every age, even if we be in our 80s and with Alzheimer’s disease, but there are things we can do to enhance our neurogenic potential along the way.

Neuroplasticity will always be somewhat enhanced by neurogenesis (after all, new neurons get given jobs to do), and we reviewed a great book about the marvels of neuroplasticity including in older age:

The Brain’s Way of Healing: Remarkable Discoveries and Recoveries from the Frontiers of Neuroplasticity – by Dr. Norman Doidge

Our brains are still developing up to the age of 25: True or False?

True! And then it keeps on developing after that, too. Now this is abundantly obvious considering what we just talked about, but see what a difference the phrasing makes? Now it makes it sound like it stops at 25, which this statement doesn’t claim at all—it only speaks for the time up to that age.

A lot of the popular press about “the brain isn’t fully mature until the age of 25” stems from a 2006 study that found:

❝For instance, frontal gray matter volume peaks at about age 11.0 years in girls and 12.1 years in boys, whereas temporal gray matter volume peaks at about age at 16.7 years in girls and 16.2 years in boys. The dorsal lateral prefrontal cortex, important for controlling impulses, is among the latest brain regions to mature without reaching adult dimensions until the early 20s.❞

~ Dr. Jay Giedd

Source: Structural Magnetic Resonance Imaging of the Adolescent Brain

There are several things to note here:

• The above statement is talking about the physical size of the brain growing
• Nowhere does he say “and stops developing at 25”

However… The study only looked at brains up to the age of 25. After that, they stopped looking, because the study was about “the adolescent brain” so there has to be a cut-off somewhere, and that was the cut-off they chose.

This is the equivalent of saying “it didn’t stop raining until four o’clock” when the reality is that four o’clock is simply when you gave up on checking.

The study didn’t misrepresent this, by the way, but the popular press did!

Another 2012 study looked at various metrics of brain development, and found:

• Synapse overproduction into the teens
• Cortex pruning into the late 20s
• Prefrontal pruning into middle age at least (they stopped looking)
• Myelination beyond middle age (they stopped looking)

Source: Experience and the developing prefrontal cortex ← _{^{check out figure 1, and make sure you’re looking at the human data not the rat data}}

So how’s the most recent research looking?

Here’s a 2022 study that looked at 123,984 brain scans spanning the age range from mid-gestation to 100 postnatal years, and as you can see from its own figure 1… Most (if not all) brain-things keep growing for life, even though most slow down at some point, they don’t stop:

Brain charts for the human lifespan ← _{^{check out figure 1; don’t get too excited about the ventricular volume column as that is basically “brain that isn’t being a brain”. Do get excited about the rest, though!}}

Want to know how not to get caught out by science being misrepresented by the popular press? Check out:

How Science News Outlets Can Lie To You (Yes, Even If They Cite Studies!)

Take care!