A rich family has always been able to buy a tutor. In 1984 a researcher measured what that's worth, and the number was so big that people have been trying to explain it ever since. A class of 25 can't give each child more than a couple of minutes a day. This page lets you work out what it would take to give every kid the rich kid's tutor.
In a normal class, each child gets about two minutes of one-to-one help a period, and one-to-one help is the thing that works best. So the job for AI is to give every child more of those minutes, on a machine the school owns, with the teacher still in the room.
Pick a classroom or build your own. Each dot is a child, filled by the one-to-one minutes they get in one period. The clock is the period, split between the children. The classroom stays in view as you work the controls.
With the teacher, one to one
With an AI tutor
Classroom:
Minutes one to one with the teacher, per child, per period
Hours a school year (180 days)
Hours a year with the AI tutor
How much more a private tutor gives in one period
What the minutes are worth: measured effects, in standard deviations
Minutes are simple arithmetic from the inputs: the period, times the share the teacher spends one to one, divided by the class. At 100 percent the teacher does nothing but go from child to child, so that's the most it could be. A school year is 180 days, typical in the United States.
Effect sizes are from the cited studies, which measured different things in different places. A standard deviation of 1 moves an average student to about the top sixth of the class. The fantasy game is Card's invention.
The two-minute problem
Take one teacher, 21 children and a 50-minute period. That's the average primary class across the OECD countries in 2024; in the United States it's 20, and 21 in lower secondary. If the teacher did nothing but go from desk to desk, each child would get under two and a half minutes. Nobody teaches like that. There's the lesson to give, the class to settle, the questions everyone needs answered at once. So the real number is lower.
Now set the class to 1. That's what a family that can pay for a tutor buys: the whole period, adjusted to one child. The last readout shows the gap: 21 times as much, before anything else happens.
Takeaway.The arithmetic sets the limit, not the teacher. Nobody can split into 21.
What one-to-one buys
In 1984 the educational psychologist Benjamin Bloom reported that students taught one to one did about two standard deviations better than students in a conventional class of about 30. The average tutored student beat 98 percent of the class. He called finding a way to do that for every child, at a price a school could pay, “the 2 sigma problem.”
The number was too good. It came mainly from two small University of Chicago dissertations, with three-week units, and the tutored students also got mastery tests and corrective feedback. It has never been replicated. Reviews of many studies find about a third of a standard deviation: 0.33 in a 1982 review, 0.37 in a 2020 review of randomized trials of tutoring programs, with bigger effects when teachers or trained aides tutor, in the early grades, and during the school day. The evidence panel in the dial puts them side by side. Tutoring works. It just isn't magic.
The research on how children learn language explains why it works. An adult talking with one child fits the talk to that child.
“One-to-one interaction gives children access to language that is adjusted to their level of comprehension. When a child does not understand, the adult may repeat or paraphrase. The response of the adult may also allow children to find out when their own utterances are understood.”
Patsy M. Lightbown and Nina Spada, How Languages Are Learned, loc 772
The psychologist Lev Vygotsky gave the space where that happens a name.
“He argued that in a supportive interactive environment, children are able to advance to higher levels of knowledge and performance. Vygotsky referred to a metaphorical place in which children could do more than they would be capable of doing independently as the zone of proximal development (ZPD).”
Lightbown and Spada, How Languages Are Learned, loc 726
A tutor keeps a child inside that zone: not so easy they're bored, not so hard they quit. A class of 21 can aim at the middle of the room. And it takes time, which is why minutes are the thing to count.
“Learning takes place gradually, as the number of links between language and meaning and among language forms are built up.”
Lightbown and Spada, How Languages Are Learned, loc 801
Takeaway.It works, it's smaller than the famous number, and we know why: someone fits the next step to one child.
The tutor in the novel
Fiction already built the perfect tutor, and it's a warning. Ender's Game is a novel many American students read in school. Ender is a boy sent to Battle School, a training station in orbit, to be made into a commander.
The fantasy game
Orson Scott Card, Ender's Game, 1985. The tutor belongs to the people running a war.
“Ender sat on his bed with his desk on his knees. It was private study time, and Ender was doing Free Play. It was a shifting, crazy kind of game in which the school computer kept bringing up new things, building a maze that you could explore. You could go back to events that you liked, for a while; if you left one alone too long, it disappeared and something else took its place.”
Orson Scott Card, Ender's Game, loc 1169
The narrator calls it the fantasy game; the officers call it the mind game. It adapts to one child, endlessly. Ender gets stuck at a puzzle called the Giant's Drink, which has no winning choice, until he attacks the Giant instead.
Meanwhile the adults are watching. Colonel Graff and his staff read Ender's play to judge his state of mind, down to asking whether he's suicidal, and they use the school to shape him toward the war they need him for.
Where it broke.The tutor was brilliant, and it worked for the people who wanted something from the child. Everything it learned about Ender, it told them.
The same book has the opposite idea in one line. Ender's brother and sister gain influence on the nets, the novel's internet, writing under adult pen names, where nobody can see who's typing.
“Every citizen started equal, on the nets.”
Card, Ender's Game, loc 2240
That's the idea under the roadmap's two novels too: Robinson's Ministry speaks for everyone, including those not yet born, and Suarez's Exchange gives every stakeholder a share. A stake for every child, not only those who can pay.
Takeaway.Whoever owns the tutor owns what it learns about the child.
A tutor the school owns
Josh Gold, who writes this site, teaches math and computer science online. He's building a tutor of this kind: one that runs on hardware the school owns, so a child's work never leaves the building and nobody outside sells what it learns. The teacher reads what the tutor saw and decides what happens next.
“Every child deserves to have their own learning be used to help them learn.”
Josh Gold
It's the same rule as on What money is a claim on: the value stays with the people who made it. A child's mistakes are worth a great deal to a tutor, because they show exactly where the next step is. They belong to the child and the teacher.
The evidence on AI tutors is new and thin, so here is all of it that this page leans on, study by study. In a World Bank trial in Edo State, Nigeria, in June and July 2024, about 800 first-year senior secondary students had six weeks of after-school English sessions with Microsoft Copilot, running on GPT-4, twice a week in school computer labs, each session opened by a teacher. Their English improved by 0.23 standard deviations. In Harvard's introductory physics course in fall 2023, 194 students each had one lesson from a custom AI tutor and one in-class active-learning lesson. They learned more from the tutor, in less time, with effects of 0.73 to 1.3 standard deviations, but the test came right after each lesson, so it says nothing about what they remembered later.
And one where it hurt. In a field experiment with nearly 1,000 high-school math students in Turkey, students given a plain chatbot did 48 percent better on practice problems, then 17 percent worse on the exam, once it was taken away, than students who never had it. A version built as a tutor, with guardrails, removed the harm but didn't produce a gain on the exam. A machine that hands over answers doesn't teach.
The novel on Josh's reading list for this idea is Neal Stephenson's The Diamond Age, another story about a one-to-one tutor. It gets its own look in a later episode.
Takeaway.Own the machine, keep the work in the building, and build it to make the child think, not to think for them.
The teacher in the room
A child doesn't learn language from a television. How Languages Are Learned tells of two hearing brothers with deaf parents. The older one, Jim, heard plenty of language from the TV and very little in conversation, and by nearly four he was well behind. His younger brother had something Jim hadn't.
“Glenn’s linguistic environment was different from Jim’s: he had his older brother—not only as a model, but, more importantly as a conversational partner whose interaction allowed Glenn to develop language in a more typical way.”
Lightbown and Spada, How Languages Are Learned, loc 768
The researchers' older view put it in terms any teacher would recognize.
“the quality and quantity of the language the child hears, as well as the consistency of the reinforcement offered by others in the environment, would shape the child’s language”
Lightbown and Spada, How Languages Are Learned, loc 471
Quality and quantity: the tutor can add quantity. The teacher still sets the quality, and knows the child. The makers' movement puts the goal in four words.
“The Piagetian idea that ‘to understand is to invent’”
Sylvia Libow Martinez and Gary S. Stager, Invent To Learn, loc 159
The same book warns what happens when data takes the teacher's place.
“Emphasis on high-stakes standardized testing, teaching to the test, de-professionalizing teachers, and depending on data rather than teacher expertise has created classrooms that are increasingly devoid of play, rich materials, and the time to do projects.”
Martinez and Stager, Invent To Learn, loc 161
A tutor that drills for the test would be one more of those. A tutor that takes the drilling off the teacher's hands gives the teacher back the time to build things with the class. Turn the AI tutor up on the dial: the teacher's minutes don't change. What changes is what they can be spent on.
What a child is worth in 2100 asks how much a child not yet born counts. This page asks what the child already in the room gets.
Takeaway.More minutes for the child means more minutes for the teacher to be a person in the room.
Ender's Game is a novel by Orson Scott Card. How Languages Are Learned is a textbook by Patsy M. Lightbown and Nina Spada; Invent To Learn is a book on teaching by Sylvia Libow Martinez and Gary S. Stager. Quotes are given word for word with their Kindle locations. This is an unofficial reading, not affiliated with or endorsed by any of the authors. Minutes are simple arithmetic from the inputs; effect sizes are from the cited studies, which measured different things in different places. No student's work or data is used on this page.
Sources
Orson Scott Card, Ender's Game (Tor, 1985). Novel. The fantasy game, loc 1169; equal on the nets, loc 2240.
Patsy M. Lightbown and Nina Spada, How Languages Are Learned, 4th edition (Oxford University Press, 2013). Quality and quantity, loc 471; the zone of proximal development, loc 726; Jim and Glenn, loc 764 and 768; one-to-one interaction, loc 772; learning takes place gradually, loc 801.
Sylvia Libow Martinez and Gary S. Stager, Invent To Learn: Making, Tinkering, and Engineering in the Classroom (Constructing Modern Knowledge Press, 2013). To understand is to invent, loc 159; testing and teacher expertise, loc 161.
Hamsa Bastani, Osbert Bastani, Alp Sungu, Haosen Ge, Özge Kabakcı and Rei Mariman, "Generative AI without guardrails can harm learning: Evidence from high school mathematics", PNAS 122(26), e2422633122 (2025); Knowledge at Wharton, coverage. Nearly 1,000 students in Turkey; practice grades 48 percent higher with a plain chatbot, exam grades 17 percent lower once it was taken away; a guarded tutor removed the harm.
OECD, Education at a Glance 2026, Table D2.3. Average class size in 2024: OECD 21 in primary and 23 in lower secondary; United States 20 and 21.