How to check us
Last reviewed 23 August 2026 · this page is revised, not evergreen
Every claim in our books carries the number of people it was tested on. Every page tells you which part of it is oversold. Including ours.
We sell books and workbooks about how a mind works, at every age from a first week to the season after work ends. That is a category full of confident sentences with nothing behind them, and the only honest way to stand apart from it is to show the working and let you audit it.
So this page hands you the tool for checking any claim about a mind — and then invites you to use it on us. If you find something here that is wrong, tell us. We date this page and revise it.
Five questions for any claim
- How many people? A finding from 42 families and a finding from 617,466 people are not the same kind of thing, and both get quoted the same way.
- Was the control group doing something, or nothing? Almost anything beats nothing. The interesting question is whether it beats an active alternative.
- Did it hold up on a bigger sample? A striking first result is a hypothesis. A replication is a finding.
- Does the person telling you own the outcome? Authors with a financial interest in a result report positive effects markedly more often. That includes us — which is why the numbers are printed rather than described.
- Is this an average, or is it about you? Almost everything in psychology is a small shift in an average, measured on people who are not you. It is still useful. It is not a promise.
A ruler for effect sizes, because "significant" means "probably not a fluke", not "big". Picture a room of thirty people. A large effect (around d=0.65) moves most of the room a noticeable step. A small one (around d=0.19) moves the room by less than you would spot standing in it — real, worth having, and nothing like a transformation.
What we tell you is oversold
These appear in our books and workbooks with the deflation printed next to them, not omitted. Our standing rule is that the word is contested, never "debunked", wherever the deflation is itself disputed — overstating a correction is a bigger credibility problem than the original myth.
| What you were told | What happened when it was tested | Our verdict |
|---|---|---|
| The thirty-million-word gap | The original finding came from n = 42 families (Hart & Risley, 1995). A later study using different sampling did not reproduce the size of the gap (Sperry, Sperry & Miller, 2019, Child Development) — and that study is itself rebutted by Golinkoff, Hoff, Rowe, Tamis-LeMonda & Hirsh-Pasek (2019), who argue it did not sample the same extremes. The direction survives; the number does not. | Contested |
| "Name it to tame it" | The brain-imaging work usually cited for it was done on adults, not on the small child the phrase is normally aimed at. We keep the practice — putting a word on a feeling helps — and drop the neuroscience gloss. | Practice yes, story no |
| The marshmallow test as a test of willpower | Later, larger analyses found much of the original association was explained by family background rather than by a child's self-control. A useful demonstration; not a measure of character. | Contested |
| "The brain finishes at 25" | A tidy number attached to a gradual, individually variable process with no biological deadline. Useful as a reminder that adolescence is longer than the law says. Not a fact about a birthday. | Not a real threshold |
| "21 days to form a habit" | Not from any habit study. The nearest real figure is a median of 66 days with a range of 18 to 254, fitted on roughly n ≈ 40 people (Lally et al.). We print the range every time we print the number. The finding worth having from that work is the other one: missing a single day did not materially damage habit formation. | Invented |
| "10,000 hours" | A re-analysis found deliberate practice explained roughly 26% of the variance in games, 21% in music, 18% in sport, 4% in education and under 1% in professions (Macnamara and colleagues; disputed by Ericsson). What survives is narrower and more useful: practice aimed at one named weakness, with a feedback source that tells you within a day whether you got it right. | Contested |
| "Always reframe it" | Troy, Shallcross & Mauss (2013, Psychological Science) found reappraisal helped with stressors people could not control and was associated with worse outcomes on ones they could. Correlational, samples in the low hundreds. It is why our adult material asks whether a thing is fixable before it offers you a way to make peace with it. | Pointed the wrong way |
| The $75,000 happiness ceiling | Later work (Killingsworth, Kahneman & Mellers, 2023, PNAS) found the plateau appears mainly among roughly the least happy 15–20% of people, not everyone. | Narrower than quoted |
| The four-circle ikigai diagram | Drawn as a "Venn diagram of purpose" by Andrés Zuzunaga in 2011 and relabelled ikigai by British blogger Marc Winn in 2014 — which Winn has publicly confirmed. It is not Japanese and it is not from the longevity research it gets attached to. The real construct (Kamiya, 1966) is smaller and more daily. In the Ohsaki study n = 43,391 adults were asked one question about it, and the answer predicted mortality (Sone et al., 2008, Psychosomatic Medicine). | Made up in 2014 |
| Blue Zones | We decline to build on them, and the fair version is that the best-known critique remains an unpublished preprint whose author is disputed. Two independent lines settled it for our purposes: Okinawa fell from 1st to 26th of Japan's 47 prefectures by 2000 and 43rd by 2020 (Poulain et al., 2024, Journal of Internal Medicine), and a researcher who co-declared Nicoya later published that post-1930 cohorts show no advantage (Rosero-Bixby, Demographic Research, 2023). | We don't use it |
| The Mozart effect · learning styles · brain-training apps | All three are widely reported as overturned and we treat them as such in our books. Full disclosure: our sourcing for these three is secondary, and we have not yet re-verified them against the primary literature ourselves. They are on our list to check, and we would rather say that than imply a rigour we have not applied. | Not re-verified by us |
Where our numbers come from
A selection, by the stage of life the book is written for. Every one of these appears in a product with its sample size beside it.
| Stage | The finding we use | Source and size |
|---|---|---|
| A baby | What predicts secure development is not never missing a cue — it is reliably coming back after you do. Healthy pairs are out of step much of the time. | Tronick, Als, Adamson, Wise & Brazelton (1978); Mesman, van IJzendoorn & Bakermans-Kranenburg (2009), Developmental Review — 80+ studies |
| A baby | Bedtime fading: record when they actually fall asleep for three nights, set bedtime to that, then walk it back. No crying involved. Limit line, printed above the protocol: infants 6–16 months whose parents already reported a sleep problem. Nothing in it applies to newborns. | Gradisar et al. (2016), Pediatrics, randomised trial — n = 43 |
| A small one | A predictable bedtime routine improves sleep outcomes. | Mindell et al. (2009), randomised — n = 405 |
| A teenager | The presence of peers changes risk-taking in adolescents in a way it does not in adults — which is why our advice is about changing the situation rather than lecturing the person. | Gardner & Steinberg (2005), Developmental Psychology |
| A teenager | Asking someone directly about self-harm or suicide does not plant the idea. | Dazzi, Gribble, Wessely & Fear (2014), Psychological Medicine — review of 13 studies |
| Teens and adults | Specific "when X, I do Y" plans move people from intending to doing — the mechanism behind every scripted first line in our books. | Gollwitzer & Sheeran (2006) meta-analysis — 94 tests, 8,000+ participants |
| Teens and adults | Between 47% and 54% of people with genuine intentions never act on them, and the failure is at getting started rather than at wanting it. | Sheeran (2002), European Review of Social Psychology |
| Adults | Self-distancing — talking to yourself in the third person — lowers rumination. A real nudge and nothing more: the pooled effect is g = 0.19. | Murdoch et al. (2023), Stress and Health — 25 experiments, N = 2,397 |
| Adults | Rumination predicts depression more strongly than the severity of the original problem — which is why our material works on the loop rather than on the content of the worry. | Nolen-Hoeksema, Wisco & Lyubomirsky (2008), Perspectives on Psychological Science |
| Adults | Burnout is an occupational phenomenon, not a diagnosis, with no validated cut-off and heavy overlap with depression. We print it as a referral trigger, never as a verdict. | WHO ICD-11 (2019), from Maslach & Jackson (1981) |
| The second half | Retirement is relief from bad work: the health benefit was largest for people leaving poor work environments and absent in those with high-grade, low-demand, satisfying jobs. Contradicting evidence printed alongside — Behncke (2012) found harms, and Garrouste (2022) concludes there is no consensus. | Westerlund et al. (2009), The Lancet 374:490–501 — n = 14,714 |
| The second half | Volunteers had lower mortality (HR 0.78 men / 0.77 women) while their non-volunteering spouses — same house, same income, same street — did not (1.01 / 1.00). That comparison is what answers the obvious objection that healthy people simply volunteer more. | O'Reilly, Rosato, Moriarty & Leavey (2017), International Journal of Epidemiology 46(4) — 617,466 individuals |
| Every stage | Purpose predicts lower mortality equally across young, middle-aged and older adults — not moderated by age or by retirement status. This is the single finding that justifies one method told at seven stages. | Hill & Turiano (2014), Psychological Science 25(7), MIDUS, 14-year follow-up — 569 deaths |
What we don't know yet
The five questions above, turned on ourselves.
- The biggest one. Purpose predicts better outcomes robustly, and purpose is probably trainable. Nothing yet demonstrates that raising purpose produces the mortality and dementia benefits seen in the cohort studies. The bridge between the two is not built, and our whole range leans on that bridge.
- Reverse causation is live in the dementia findings: very early undetected disease could itself lower reported purpose.
- Serve-and-return as a five-step sequence is packaging around a real phenomenon. The phenomenon is well evidenced; the numbered protocol is ours, and it has not been tested as a protocol.
- Motivational interviewing works, and only a little — around d = 0.17 in the trials we rely on.
- The retirement literature has no consensus, and we say so on the page rather than picking the study that suits the book.
- The monitoring and disclosure literature is correlational. Well-adjusted teenagers also disclose more, so we cannot tell you which way the arrow points.
- Three of our myth entries are not re-verified by us (see the table above).
What we will never claim
- That a book will change anyone's mental state. Our products are named for situations — "for when your teen's gone quiet", "for the hour you didn't mean to spend" — never for outcomes. Nobody can prove an outcome claim about a mind, so nobody should be allowed to make one, including us.
- That any of it is quick. Almost nothing here works on the first go.
- That we have reviews when we do not. As of today we have published none, because nobody has yet sent us one we can print with their name on it and their permission.
- A "was" price we never charged, a countdown to a date we invented, or a stock limit on a book that is printed to order and cannot run out.
When a book is the wrong instrument
Finding the right line for your country…
Persistent withdrawal alongside loss of interest, or a change in sleep or appetite, warrants a GP rather than a worksheet. Asking someone directly about self-harm does not plant the idea — it gives them somewhere to put it.
Tell us we're wrong
If a citation here is misused, a sample size is misstated, or a finding has been overturned since we last reviewed this page, write to hello@masterfulmindsbooks.com. We will correct it and date the correction. That is the whole point of publishing this.