The Promised Neverland- Why 12 years old?

The Promised Neverland

This is not the first time I’ve looked into the world of The Promised Neverland, as I have been a fan of the manga since it came out in 2016.  It was not until recently, however, that I was able to sit down and watch the anime.  This was in part due to work keeping me busy, Crunchyroll was not really working for me, and I had read the manga so it wasn’t like I was missing part of the story.  Needless to say, I was absolutely floored after watching the first 4 episodes at Otakon 2019.  Cloverworks absolutely nailed it, from the characters, plot, and general ambiance of the show.  In a few places I think the anime actually did it better than the manga.  I couldn’t have asked for a better adaptation.  However, after watching it a question that has been lingering in my subconscious came back into my head.

Why are the highest quality kids harvested at 12 years old?

I’m sure I am not the only one who has had this question.  If I am being honest, the most obvious answer would be how much lower the horror and drama would be if it were older teenagers or adults being harvested instead of young kids.  The second answer would probably be cultural in that Japanese kids leave elementary school at 12 years old.  This further nails home the point of just how young the children actually are to the viewer.  However, I have a different answer to the question of why the highest quality kids are harvested at 12 years old: Brain development, more specifically brain size.

Warning- Manga Spoilers

Later on in the manga we learn that the demons evolve via horizontal gene transfer, which I have covered previously.  To summarize, the demons gained human intelligence by eating humans, specifically their brains, which is the most prized part of the human body to them.

The Promised Neverland

Brain size

Brain size can be measured 2 ways, by volume, or by mass.  While it might not seem like much, it is rather important for today’s discussion.  Brain volume, or the total space the brain takes up, increases with age, peaking around age 40.  A major component of this is the increase in the size of the ventricles or fluid filled spaces inside the brain, which are the purple sections in the diagram below.


If you remember that the demons prize the brain and need the DNA in the brain cells, they would want a heavier brain with more cells and not a bigger brain with more empty space.  Thus, we will be looking at brain mass.

Gray Matter vs. White Matter

Before diving into how brain mass changes with age we need to go over two basic types of brain tissue, gray matter and white matter.

Gray Matter and White Matter

The gray matter contains several different types of cells, but the important one for today is the neuronal cell body, or the main mass of a nerve cell containing the nucleus. (FYI- the nucleus contains the cell’s DNA.)  The white matter contains the axons or long extensions of the nerve cell connecting it to other nerve cells, and it does not contain DNA.  The demons of The Promised Neverland need the nerve cell DNA to keep their human form and intelligence, thus they are going to want more gray matter over white matter.

With that taken care of we can finally examine how brain mass changes with age.  As seen in the graph below, brain mass for both males and females peaks between 16-20 years old.

brain weight by age

This is not the end of the story, however, because the mass of gray matter and white matter peaks at different ages.

brain size by age

I do realize that the graph compares the volume of gray matter in normal individuals to the amount of gray matter in individuals with autism, however that is not important.  What we can see is that the amount of gray matter peaks just before 12 years of age.  (FYI- it is backed up by data but this was the best graph I could find on the topic).  This also means that the brain at age 12 contains the most DNA on average, which the demons need to eat in order to retain their human form and intelligence.


Thus, it makes sense that the demons would harvest the highest quality kids or brains at 12 years of age when the amount of gray matter is at its highest and contains the most DNA.  Now I do realize that there are additional reasons to harvest the kids at 12 years old, mainly puberty, as it would introduce a number of mental and physical changes that would make raising them past age 12 more difficult.  Now I am torn on whether or not the author got it right by accident or knows something about brain development.  My first instinct is that he got lucky, but he does bring up horizontal gene transfer, which is a fairly advanced topic.  This is something I do not cover in my classes, and only someone with advanced biological knowledge has probably heard about.  What do you guys think?

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