Spurious Alpha

Everyone has a theory about what moves the market — the moon, Mondays, Mercury in retrograde, the shape of a chart. Most of them take about ten seconds to test against real prices, and when you do, something strange happens. They all seem to work. Buy the S&P only on prime-numbered dates and you'd have made money. Buy it only when a coin says so and you'd have made money. Full moons, Tuesdays, dates with a 7 in them — money, money, money.

Try it below. Stake some money, pick a superstition, and watch two real years of the market reward it. Then find out what doing nothing would have done instead.

Why every hunch "works"

There are three ways to fool yourself.

1 — Spurious correlation. Two unrelated things line up by chance. Tyler Vigen has collected hundreds: the number of people who drown in swimming pools each year rises and falls with the number of films Nicolas Cage releases. Nobody thinks one causes the other.

2 — Spurious regression. Two things that both grew over the same years can look linked even when neither touches the other — the S&P 500 and the price of a house both roughly tripled since 2010. To tell a real relationship from a coincidence you compare how they move week to week, not how high each has climbed. Compare the heights, and everything that rose together looks connected.

3 — The missing benchmark. A rising tide floats all boats, and a rising market floats every superstition with it. While stocks climb, any rule that keeps you invested part of the time makes money — the moon, Mondays, a coin flip, they all "work." The only honest way to judge a rule in a trending market is against the one line you never drew: buy-and-hold. Leave that line off the chart, and a guess looks exactly like a forecast.

The third is the whole point of the tool. The original full-moon post shows a moon strategy climbing across a clean stretch of bull market. It's true, and it's empty: over that same window, eleven of the thirteen superstitions made money, and 58 of 60 purely random rules did too. The market rose 48%. Nothing invested could have failed.

So the moon strategy that started all this? In the window everyone was posting about, it ranks fifth of thirteen. Run it against the full decade and it slips to eighth — up 64% while the market returned 303%. It never once beats doing nothing.The tool uses real SPY total-return prices from Yahoo Finance, 2015–2025. The headline window is January 2024 – December 2025, when the market rose 48%. That it rose is the point, not a flaw: the illusion only appears while the market is rising, which is why nobody posts moon-phase charts after a bad year. Moon phase is computed from a 29.53-day cycle; no transaction costs, which only makes the hunches look worse.