C15 Learning by teaching
narrowed 1 attack: 1 narrowed · serves Q1 · last change 25 October 2026
Explaining newly learned material to others (actually teaching it, not just preparing to) improves learners' own understanding on a delayed test more than studying it for themselves when learners studied the material expecting to teach it; a lasting gain is not established for a short explanation added after ordinary study.
In practice
If learners are going to explain material to someone else, tell them before they study it, then have them actually explain it, ideally to someone who can ask questions back. In the studies on file, a benefit on later tests was shown when learners studied expecting to teach and then taught. A short explanation added after an ordinary lesson helped on a test right afterwards, but in a large classroom study it left no clear advantage eight weeks later.
Go deeper
Where it holds
Main limits
- About the teacher-learner's own learning, not about being tutored (C5, C10).
- Evidence for the later benefit mostly from lab studies with university students who studied expecting to teach (Kobayashi 2018; Fiorella & Mayer 2013).
- Interactive teaching helped more than non-interactive (Kobayashi 2018).
- Not established for a short explanation added after ordinary study: in three classroom experiments with 1,074 German grade 7–8 physics students, 15 minutes of explaining a lesson by voice message to an imagined peer beat restudying right afterwards but not eight weeks later (difference not significant; a later gain only for students with low or medium interest in physics; Lachner et al. 2025).
- A meta-analysis found no benefit from teaching when learners had not studied expecting to teach (Kobayashi 2024; abstract only).
All limits (11)
Inside the circle: the core of the claim. Outside: every bound and carve-out in the claim file, numbered, with the attack that forced or added it where the record shows one.
forced by a narrowing attackin the original wordingwritten at creation, or added without a narrowing
Warm cards in the list were added by later attacks; plain cards were in the original draft or written at creation.
The learning of the person who explains, compared with studying for themselves; outcome delayed tests of the explained material; not the effect on the tutee (C5, C10) or peer discussion of concept questions (candidate K38).
Written when the claim was created (16 October 2026); not forced by an attack
Evidence on file: Kobayashi 2018 (Japanese Psychological Research; meta-analysis of 28 studies; Hedges' g 0.35 for preparing to teach and 0.56 for teaching after preparing; benefits also after a delay; larger for interactive teaching); Fiorella & Mayer 2013 (Contemporary Educational Psychology; actually teaching after preparing helped on a delayed test about one week later).
Written when the claim was created (16 October 2026); not forced by an attack
Expectancy and durability bound (added 2026-10-25, nightly attack): In three classroom experiments with 1,074 German grade 7–8 physics students, 15 minutes of explaining a lesson by voice message to a fictitious peer, with the materials at hand, beat 15 minutes of restudying on a test right afterwards but not on a test eight weeks later (difference not significant; a later advantage only for students with low or medium interest in physics) (Lachner et al. 2025).
Forced by attack #1 · Lachner, Russ, Hübner, Sibley & Scheiter 2025 · 25 October 2026
Wording changed later: Forced by attack #1 · Lachner, Russ, Hübner, Sibley & Scheiter 2025 · 25 October 2026.
Lachner, Russ, Hübner, Sibley & Scheiter 2025 (Educational Psychology Review 37, 88; open access, CC BY; full text read on the publisher's site): preregistered one-stage individual-participant analysis of three analogous classroom experiments (project GRAIL) with 1,074 German grade 7–8 students in 11 schools; after the same 45-minute curriculum lesson on the converging lens, students were randomly assigned to 15 minutes of non-interactive teaching (recording a voice message to a fictitious peer, Mia, who had stated two common misconceptions; materials allowed, not to be read aloud) or 15 minutes of restudying the materials with notes.
Forced by attack #1 · Lachner, Russ, Hübner, Sibley & Scheiter 2025 · 25 October 2026
Because assignment came after the lesson, the teaching group had not studied expecting to teach.
Added on 25 October 2026 when the claim was reworded (not an attack)
Scientific knowledge (15-item ROC-CI test, α = .72): immediate posttest β = 0.23, p < .001; delayed posttest eight weeks later β = 0.09, p = .110 (not significant); interest in physics moderated the delayed effect (β = −0.18, p = .043; low- and medium-interest students gained, high-interest students did not).
Added on 25 October 2026 when the claim was reworded (not an attack)
Of the three sub-studies, Study 1 found no long-term effect and Study 3 (under review) no effect even immediately.
Added on 25 October 2026 when the claim was reworded (not an attack)
The authors attribute the missing delayed effect to the short, single, open-book session and leave open whether repeated or closed-book teaching would last.
Added on 25 October 2026 when the claim was reworded (not an attack)
Kill withheld: C15's kill-condition is about teaching after preparing to teach, and these students had not studied expecting to teach; the delayed estimate points the same way as the claim and was not shown to be zero (no equivalence test); one topic, one 15-minute dose.
Added on 25 October 2026 when the claim was reworded (not an attack)
The evidence for a delayed benefit on file comes from studies in which learners studied expecting to teach (Fiorella & Mayer 2013; Kobayashi 2018).
Added on 25 October 2026 when the claim was reworded (not an attack)
Consistent with this, Kobayashi 2024 (Educational Psychology Review 36, 26; meta-analysis of 39 studies; abstract only, paywalled) reports that teaching after studying with teaching expectancy beat studying without it, g = 0.48 [0.34, 0.63], while teaching after studying without teaching expectancy did not, g = −0.02 [−0.14, 0.11]; whether those outcomes were delayed is not stated in the abstract.
Added on 25 October 2026 when the claim was reworded (not an attack)
2 of 11 bounds and carve-outs were forced by a specific attack.
How it changed
The ledger records 1 attack, but no earlier wording of C15 is on record, so the text cannot be replayed step by step. Only the current wording is shown.
Explaining newly learned material to others (actually teaching it, not just preparing to) improves learners' own understanding on a delayed test more than studying it for themselves when learners studied the material expecting to teach it; a lasting gain is not established for a short explanation added after ordinary study.
What would change this
What would kill it: A large preregistered experiment or meta-analysis finds that teaching others after preparing gives no better delayed-test learning than studying alone for the same time. Added as a claim on 16 October 2026, promoted from a candidate belief in the evidence library.
Attack record
Reworded on 25 October 2026 (a change of wording, not an attack, so it is not counted). Shortened the claim sentence to the plain rule. The Lachner et al. 2025 study detail moved unchanged into limits; the Kobayashi 2024 detail was already there. No new numbers; status, survivals and kill-condition unchanged.
1 attack: 1 narrowed. Counts of attacks, not a score.
Full attack list (1)
In a preregistered analysis of three randomised classroom experiments with 1,074 secondary-school students, explaining a just-taught physics lesson to a fictitious peer for 15 minutes beat restudying for the same time on an immediate test but not eight weeks later (β = 0.09, p = .110), so teaching others does not reliably improve the teacher's own understanding on a delayed test more than studying alone.
Strongest reading and verdict reasoning
Steelmans to: C15's sentence covers any actual explaining of newly learned material to others, with no condition on how learners studied or how long they taught; this is the largest test on file (about ten times the lab studies the claim rests on), in real classrooms, randomised, preregistered, with equal time on task and a long delay, and its delayed advantage was small and not significant, matching Kobayashi 2024's meta-analytic finding (abstract) that teaching after studying without teaching expectancy gives no benefit (g = −0.02).
Why: Nightly-attacks 2026-10-25 Europe/Stockholm, widened-search slot (C15, four consecutive fruitless searches). DOI checked on Crossref (title, five authors, Educational Psychology Review 37, article 88, published 2025-08-28) and Unpaywall (open access, CC BY, publisher version). Full text read on the publisher's article page (the PDF is behind a bot check from the box; not stored). Design: project GRAIL, three analogous experiments, German grade 7–8 physics, 11 schools; preregistered re-analysis (AsPredicted 26p4-7vsh); same 45-minute lesson on the converging lens (15-minute introduction plus 30 minutes of experiments) taught by one certified physics teacher; then random assignment to 15 minutes of non-interactive teaching (voice message to a fictitious peer responding to two misconceptions; materials allowed) or 15 minutes of restudying with notes. Scientific knowledge (ROC-CI test, 15 items, α = .72): immediate β = 0.23, p < .001; eight weeks later β = 0.09, p = .110; interest moderated the delayed effect (β = −0.18, p = .043). Sub-studies: Study 1 no long-term effect, Study 3 no effect immediately or delayed. Kill withheld: the teaching group had not studied expecting to teach, which C15's kill-condition requires; the delayed estimate is positive and not shown to be zero; one topic, one short open-book session (the authors' own explanation for the null). Not a restatement: the Bounds named no condition on teaching expectancy or dose, and open question 13 concerns written versus spoken teaching (here spoken). Not a duplicate (first ledger entry for C15). Peer reviewed; not a preprint. Status alive → weakened. Claim and Bounds rewritten same turn; brief, practical rule, limits and headline and the C15 front-page line rewritten.
Evidence
9 linked sources: 3 supports, 4 limits, 2 challenges (first-pass draft, pending human review). supports: Kobayashi 2018, Fiorella & Mayer 2013, Lim et al. 2021 · limits: Koh et al. 2018, Wong & Lim 2026, Hoogerheide et al. 2016, Lachner et al. 2021 · challenges: Kobayashi 2024, Lachner et al. 2025