The thesis
“Putting $500 a month into the S&P 500 since 2010.”
How buffet read it
$10,000 initial investment in SPY, plus $500 added every month.
2010–2026.
Dividends reinvested.
Benchmark: S&P 500.
SPY · 4,189 trading days · benchmark SPY
$10,000 at the start, then $500 a month — $109,500 in altogether — became
$0
This thesis is the S&P 500, so it returned what the index returned over 2010–2026: $480,342. There is no gap to explain — the question is whether you would have sat through the fall below.
The worst stretch
−33.7%, February 2020 to March 2020 — you would have sat through one month of that, and it took until August 2020 to get back to level.
the S&P 500 fell 33.7% at its worst over the same period. The thesis moved 17.1% a year against 17.1% for the index.
Where the thesis was wrong
The portfolio returned +338.7% on money put in over 2010–2026, at +14.2% a year, because it is the index — SPY tracking the S&P 500 exactly. The worst stretch was a −33.7% drawdown from 19 February 2020 to 23 March 2020, recovering by 10 August 2020, which is 33 days of losses that required holding through without stopping contributions. 2022 gave back −18.2% and 2018 gave back −4.6%, both years where continuing the $500 monthly addition was the only thing the thesis asked of you. The $109,500 put in over 16.6 years grew to $480,342, but that figure depends entirely on not missing months, not pausing in downturns, and the lump-sum assumption of $10,000 at the start — change any of those and the number changes.
Counter-test
Same money, same schedule, parked in 1–3 month T-bills instead: $131,938.
Year by year
| Year | The thesis | SPY |
|---|---|---|
| 2010 | +13.1% | +13.1% |
| 2011 | +1.9% | +1.9% |
| 2012 | +16.0% | +16.0% |
| 2013 | +32.3% | +32.3% |
| 2014 | +13.5% | +13.5% |
| 2015 | +1.3% | +1.3% |
| 2016 | +12.0% | +12.0% |
| 2017 | +21.7% | +21.7% |
| 2018 | −4.6% | −4.6% |
| 2019 | +31.2% | +31.2% |
| 2020 | +18.4% | +18.4% |
| 2021 | +28.7% | +28.7% |
| 2022 | −18.2% | −18.2% |
| 2023 | +26.2% | +26.2% |
| 2024 | +24.9% | +24.9% |
| 2025 | +17.7% | +17.7% |
| 2026 | +13.4% | +13.4% |
What this verdict does not include
- US-listed stocks and ETFs only, priced on end-of-day closes.
- No tax, no trading fees, no slippage, no bid-ask spread.
- Dividends are reinvested, because closes are adjusted for them.
- Survivorship is not corrected for: only instruments that exist today are held.
- SPY is used as the S&P 500 vehicle; VOO or IVV would produce near-identical results.
- An initial lump sum of $10,000 is assumed at start since no opening amount was stated; adjust if you started from zero.
Keep this thesis
buffet re-runs it once a month and mails you the new number. Nothing else is ever sent.
Another thesis
Every verdict names something it could not test. That gap is usually the next thesis.
Run another thesis — $5