There is a book called The Delinquents Who Cannot Cut a Cake.
The author, child psychiatrist Koji Miyaguchi, reports that many of the delinquent boys he met cannot perform a task as simple as cutting a round cake into three equal pieces.
His point: weakness in cognitive function ties directly to difficulty adapting to society.
This is not only a problem of delinquent boys. The same structure shows up in parenting too.
What “Cannot Cut a Cake Evenly” Actually Means
Cutting a cake into thirds. To an adult it looks trivial.
But this seemingly trivial task quietly draws on:
understanding the concept of “equal portions,” the ability to mentally divide a circle into three, and the ability to compute where to put the knife —
several cognitive functions at once.
Some people are weak in this “invisible cognitive layer.”
The diagnostic gray zone between intellectual disability and normal intelligence is sometimes called “borderline intellectual functioning.”
In IQ terms, around 70 to 84. It does not qualify a person for a disability certificate, but it leaves them tripping over things at every step of social life. Roughly 14% of the population is estimated to fall into this range※1.
Mothers Who Can’t Make It to Prenatal Checkups, Mothers Who Can’t Quit Smoking
In parenting-support settings, you encounter mothers who fall outside what most people consider normal.
They don’t go to prenatal checkups. The infant health-check notices arrive, and they don’t go. They know they should quit smoking and can’t. They cannot keep track of their child’s vaccination schedule.
From outside, this gets read as “laziness” or “irresponsibility.”
It also gets called neglect. The end result is that the child is not getting the care they need, so the call is not exactly wrong.
But turn your attention to what is happening inside the mother, and a different scene comes into view.
She reads the checkup notice and cannot hold the date, place, and required items in her head at the same time.
She looks at the vaccination schedule chart and cannot tell which one to give next.
If a clinician does not have much experience, the intellectual disability goes unrecognized — and so it is never diagnosed.
From those around her, she gets treated as “an ordinary adult.”
And as an “ordinary adult,” she is asked to take on adult responsibilities. When she cannot, the label “irresponsible parent” gets attached.
A Question of “Capacity,” Not “Malice”
Abuse comes in several variations:
Parents who tie the child down with control and surveillance.
Parents who cannot regulate their anger and end up striking the child.
Parents who simply lack the capacity to deliver the care a child needs.
Parents in this third group sometimes have no intent to harm the child at all. Many are, in their own way, trying their hardest.
The trouble is that “their hardest” doesn’t reach the level required to raise a child.
For example: weakness in inferring why the child is crying. The work of holding three possibilities — “is it hunger, is it sleep, is it a wet diaper?” — in mind at once and trying them in sequence is genuinely hard for them.
The result is that they stand frozen in front of a crying child. Or only their own feeling — “the noise” — registers, and they shout.
Without seeing this structure, responding only with “that parent is awful” or “poor child” doesn’t solve anything.
No amount of scolding or instruction changes the parent’s underlying cognitive capacity.
Parents Carrying an Invisible Difficulty
The difficulty borderline-intelligence parents carry is hard to see from the outside.
Conversation works. There are smiles. They can even look like “a together kind of person.”
But the small details of daily life —
submitting paperwork before a deadline, having meals ready at the same time each day, picking up that the child’s condition has changed — all of those “ordinary” things sit on a tightrope.
What is needed is not blame. It is forms of support designed for these specific cognitive traits.
One counselor put it this way: “Supporting people in the borderline range is hard not only because they don’t realize they are struggling — but because they cannot put what they are struggling with into words.” There is a vague unease of living. But that unease never quite reaches the recognition “there is something about my own cognition.”
So they do not show up at support desks on their own; and even when they do show up, they cannot describe what is wrong. The reason their difficulty so often only surfaces after abuse of a child has already been discovered is exactly this.
What the Child Beside Such a Parent Was Seeing
The parent is not aware of their own cognitive limits. But that means the child, too, was looking at “something with no name.”
“Why does my parent never quite follow what I’m saying?” “Every time we talk, the story gets twisted.” “Their emotion just suddenly explodes — I have no idea why.”
— Children stack up these experiences and, in most cases, search for the cause inside themselves: “the way I said it must have been wrong,” “I’m just not handling it well.”
A parent with borderline intelligence or mild intellectual disability “can hold a conversation,” “smiles,” “looks at first glance like an ordinary parent.” That is exactly why the child grows up taking the parent’s traits as “my parent’s personality” or “the way our house is.” Without a chance to compare with the world outside, they don’t even register that this is not normal.
References
※1 The population share of borderline intellectual functioning (IQ 70–84) is a theoretical figure (about 13.6%) under the standard statistical assumption that IQ follows a normal distribution with mean 100 and standard deviation 15. The same figure is cited in Koji Miyaguchi, The Delinquents Who Cannot Cut a Cake (Shincho Shinsho, 2019) and Children with Borderline Intellectual Functioning (SB Shinsho, 2021).
