What Adam Is Reading
The Coin Was Never Fair
A viral thread on the mathematics of luck gets four of its five ideas right, cites the one man who disproved its opening premise, and never notices.
Multi-source synthesis · 13 sources · STAGED FOR REVIEW · August 10, 2026

There is a sentence in the middle of this thread that does more damage than its author intended. Describing why people cannot fake a random coin sequence, he writes that five heads in a row feels wrong "even though a real coin produces exactly that constantly." Two paragraphs later he introduces Persi Diaconis, the Stanford mathematician who left high school at fourteen to tour with the sleight of hand master Dai Vernon, as the authority on all of this.

Diaconis is the authority. That is the problem. He is also the man who spent twenty years demonstrating that a real coin does not do what the sentence says it does. A vigorously flipped coin lands the same side it started about 51 percent of the time, and in 2023 a team of forty eight people flipped coins 350,757 times and found 50.8 percent, which is one of the better calls in the history of applied probability.

The thread is titled "The Hidden Rules of Randomness." It is a good thread. It is right about the central limit theorem, right about human pattern generation, right about bijections, and mostly right about Renaissance Technologies. What follows is the layer by layer audit, plus the two things it left out that are more interesting than anything it included.


The five layers, audited
1
Chaos has a shape. Coin flips pile up into a bell curve.
What holds

Entirely correct, and the framing is good. The crudest binary process, summed, converges on the same smooth normal curve you get from processes that could land anywhere. That is the central limit theorem, and the observation that one shape keeps reappearing in finance regardless of what the underlying pieces look like is the right takeaway.

Where the dating slips

He calls it "a coin flip formalized over a century ago." Abraham de Moivre published the normal approximation to the binomial in 1733. Laplace generalized it in 1810. The Lindeberg and Lévy statement most people learn arrived in the 1920s. "Over a century" is true the way "over a dollar" is true of a hundred dollar bill. Say three centuries and the point lands harder, because the whole thesis is that the edge comes from very old free ideas.

Solid
2
The worst random number generator is you.
What holds

The biography checks out. Diaconis, born 1945, left George Washington High School at fourteen to tour with Dai Vernon, took a BS in mathematics from City College of New York in 1971, a Harvard PhD in statistics in 1974, and went straight to Stanford. The underlying claim holds too. Asked to produce a random binary string, people alternate too often and truncate runs, a finding that has survived fifty years of replication.

What he does not know he is standing on

The single most cited demonstration of "humans invent patterns in noise" is the 1985 Gilovich, Vallone and Tversky basketball paper that gave us the hot hand fallacy. In 2018 Joshua Miller and Adam Sanjurjo showed that paper contains a subtle selection bias. Counting substreaks inside a finite sequence is not the same as sampling from it, and the naive estimator is biased downward. Correct for it and the hot hand comes back at roughly 13 percentage points.

So the flagship study proving that smart people see patterns in randomness was itself a case of smart people misreading a random sequence. That is the best joke available in this entire subject and the thread walks past it.

Solid
3
There is a formula for how random something actually is, and it came from card tricks.
What holds

Dave Bayer and Persi Diaconis, Trailing the Dovetail Shuffle to its Lair, Annals of Applied Probability, 1992. They model the riffle shuffle, derive the probability of any arrangement from the count of rising sequences, and measure distance from a truly random deck in total variation distance. The numbers for a 52 card deck are the reason "seven shuffles" entered the culture.

Shuffles12345678910
Distance from random1.001.001.001.00.924.614.334.167.085.043

Nothing happens for four shuffles. Then the deck falls off a cliff between five and seven. That abruptness is the cutoff phenomenon, and it generalizes to three halves of log base two of n.

The load bearing claim he cannot support

He writes that "the same formula quant desks use to check whether a price sequence has actually lost its structure" traces back to shuffling. Total variation distance and mixing time are the standard currency of Markov chain Monte Carlo convergence diagnostics, which is genuinely everywhere in quantitative finance, but "the formula quant desks use to test a price series" is not a thing that exists. Desks test serial structure with autocorrelation, variance ratios, Ljung and Box, Hurst exponents, entropy estimators. The lineage he is gesturing at is real and the specific claim is invented.

Mostly Solid
4
Impossible counting problems are usually wearing a disguise.
What holds

Correct and cleanly explained. The example is stars and bars, the standard bijection between distributing n identical balls into k labeled boxes and arranging n stars with k minus 1 dividers in a row. The count falls out as n plus k minus 1, choose k minus 1. The graphical device goes back at least to Ehrenfest and Kamerlingh Onnes in 1914, and the name was popularized by William Feller in An Introduction to Probability Theory and Its Applications, first edition 1950.

Minor

"A counting trick older than modern finance itself" undersells it again. Bijective counting is as old as combinatorics. This is the one layer where nothing needs fixing except the framing.

Solid
5
Renaissance found the data first and asked why afterward.
What holds

The characterization of the Medallion method is accurate and the returns are real. Roughly 66 percent annualized before fees from 1988 through 2018, with no negative year in that window. One hundred dollars in 1988 becomes about 398.7 million dollars in 2018 on the gross figures. Gregory Zuckerman's The Man Who Solved the Market is the standard account, and Bradford Cornell has a paper arguing the record "stretches explanation to the limit," which is a polite way of saying nobody has a satisfying story for it.

The number that breaks the moral

The thread's lesson is that a pattern repeating thousands of times independent of any story beats a pattern you went looking for. Fine. But Renaissance runs funds that outsiders can buy, built by the same people on the same philosophy. In 2020 Medallion returned 76 percent. Through December 25 of that same year the Renaissance Institutional Equities Fund was down 22.62 percent and Renaissance Institutional Diversified Alpha was down 33.58 percent.

Same firm. Same data first religion. Roughly a hundred point spread. Whatever Medallion has, it is not the philosophy, because the philosophy is available to the public funds and the public funds lost money. The thread cites Renaissance as proof that method beats narrative, and Renaissance is the cleanest available demonstration that method alone is not the thing.

Mostly Solid

The coin, which nobody checked

Every layer in that thread rests on an idealized coin. Layer one uses it to build the bell curve. Layer two uses it to shame the reader for not being able to fake it. And the whole edifice is introduced by the man who took the coin apart.

In 2007 Diaconis, Susan Holmes and Richard Montgomery published Dynamical Bias in the Coin Toss in SIAM Review. A flipped coin does not spin about a fixed axis. Its normal vector precesses around the angular momentum vector, so the coin spends more of its flight with its starting face up than you would guess. Filming half dollars at 600 frames per second, they got an estimated same side probability with a mean of 0.508, which they rounded to 51 percent. If the precession angle stays under 45 degrees the coin never turns over at all.

In 2023 František Bartoš and colleagues went and did it. 350,757 flips from 48 tossers. Same side rate 0.508, with a 95 percent credible interval of 0.506 to 0.509. The prediction was 0.51. That is a physics paper calling a behavioral result to within two thousandths.

The part that matters. The bias is not toward heads. Pr(heads) came back at exactly 0.500, interval 0.498 to 0.502. The coin is perfectly fair about which face it prefers and quietly unfair about which face you started with. A person watching for a heads bias would run that experiment forever and find nothing, because they were auditing the wrong axis.

Which is a better ending than the one the thread wrote. He closes by asking whether you have ever actually checked if your pattern was real. The sharper question is whether you checked the right variable. Diaconis did not find the coin bias by flipping more coins. He found it by filming one.


What this looks like from a dialysis unit
Editorial note. This section is Adam's, not the source's. The thread is about finance and says nothing about medicine.

Medicine runs the same three errors, on people.

We alternate too often. A unit has three access infections in a month against a baseline of one and a half, and a root cause analysis convenes. Poisson arithmetic says a run like that shows up in a quiet unit with unremarkable frequency. We investigate the run and not the rate, which is the substreak problem in scrubs.

We never watch the deck stop moving. Bayer and Diaconis found that four shuffles do nothing and the fifth through seventh do everything. Quality programs almost never ask how many cycles a change needs before the signal is real, so we read month two of a twelve month intervention and either kill it or declare victory. Both are the same mistake made at different points on the curve.

And we audit the wrong axis. Every dialysis quality metric in the ESRD program is a heads count. Adequacy, vascular access type, hospitalization rate, standardized mortality. All of them measure the face that came up. None of them measure the face the patient started with, and the starting face is the entire game. A unit whose patients arrive crashing into hemodialysis with a catheter and no nephrology follow up is not flipping the same coin as a unit whose patients arrive with a fistula placed nine months earlier. We have spent twenty years refining our measurement of the outcome and comparatively little on the precession.

Diaconis needed a high speed camera to see it. We mostly need to admit the camera would be worth buying.

So What

The thread's argument is that the edge lies in knowing whether a pattern is real. The stronger version, which its own sources support and it never states, is that the edge lies in knowing which variable to test. The hot hand study was wrong because it counted the wrong thing. The coin looks fair because everyone counted heads. Renaissance's public funds lost a quarter of their value running the correct philosophy. In all three cases the method was fine and the question was aimed slightly off.

Confidence. The five layers were verified against primary sources, four of them fetched directly. The one claim I could not verify and believe to be invented is that a specific shuffling derived formula is standard on quant desks. The dialysis section is argument, not evidence, and should be read as such.

Sources

Original thread: MindArch (@mindarchx), "The Hidden Rules of Randomness: The Math Behind Luck," X, August 3, 2026. x.com/mindarchx

Shuffling: Bayer D, Diaconis P. Trailing the Dovetail Shuffle to its Lair. Annals of Applied Probability 1992;2(2):294–313. PDF

Coin physics: Diaconis P, Holmes S, Montgomery R. Dynamical Bias in the Coin Toss. SIAM Review 2007;49(2):211–235. PDF

The 350,757 flips: Bartoš F, et al. Fair coins tend to land on the same side they started. JASA 2025 (arXiv 2310.04153, 2023). arXiv · JASA

Hot hand correction: Miller JB, Sanjurjo A. Surprised by the Hot Hand Fallacy? A Truth in the Law of Small Numbers. Econometrica 2018;86(6):2019–2047. Wiley · plain language walkthrough at Data Colada 88

Diaconis biography: MacTutor History of Mathematics archive, University of St Andrews. Entry · Quanta profile, 2015

Medallion returns: PWL Capital, "Renaissance Technologies Medallion Fund: An Exception to the Indexing Rule." Analysis · Cornell B, via Institutional Investor

The public funds: "Renaissance's Medallion Fund Surged 76% in 2020. But Funds Open to Outsiders Tanked." Institutional Investor, January 2021. Article

Central limit theorem history: de Moivre–Laplace theorem. Overview · From classical to modern central limit theorems

Bijection: Stars and bars, after Feller, An Introduction to Probability Theory and Its Applications, first edition 1950. Overview