After a hot inflation month, the S&P 500 has fallen 49.5% of the time — against 34.8% after every other month. That's the headline result of a study by TradingView analyst SignalPilotLabs, which checked 676 months of US data going back to April 1970. October's CPI print lands on 10 November at 13:30 UTC, and the numbers say it deserves attention.

What counts as a 'hot' month?

The bar is simple. A hot month is one in which consumer prices rose 0.4% or more month on month. The study uses the CPI for all urban consumers, seasonally adjusted (FRED:CPIAUCSL), set against the S&P 500's monthly closes (SP:SPX), from April 1970 to September 2026.

Out of 676 months, 214 qualify as hot. September 2025 was one of them: prices rose 0.66%, well over the line. For each hot month, the analyst asked two questions written down in advance — did the S&P 500 fall that same month, and did it fall the following month, when the hot number became public? Pre-registering the test matters. It rules out the temptation to sift the data for a pattern that fits.

What did the study find?

The month-after effect is the strong one. After a hot CPI month, the S&P 500 fell 106 of 214 times — 49.5%. After all other months, it fell only 34.8% of the time. The typical month following a hot one was flat, +0.01%, against +1.30% after the rest.

The same-month signal is weaker and unstable. In the hot month itself, the index fell 45.3% of the time versus 37.0% for other months — but since 1998 that relationship flipped, 29.6% against 40.1%. So the market's reaction inside the hot month tells you little. The month after is where the record holds.

One caveat on the data: October and November 2025 are excluded, because October 2025's CPI was never published during the government shutdown.

Could it just be luck?

The analyst stress-tested the result. Drawing 214 months at random, 10,000 times over, produced 106 or more falls in only 2 draws. Long odds. The pattern also held in both halves of the record:

• 1970 to 1997: the month after a hot CPI print fell 51.0% of the time, versus 33.7% after other months.
• 1998 to 2026: 46.5% versus 35.7%.

That consistency is what separates a real statistical signal from noise that happens to look like one.

When has this played out?

The hottest month on record in this dataset is August 1973, when prices rose 1.81%. The S&P 500 fell 3.67% that month — then rose 4.01% the next, a reminder that a tendency is not a rule.

In 2026, four months ran hot:

• March: +0.85%
• April: +0.64%
• May: +0.48%
• September: +0.66%

After September's print, October fell 0.93%. August came in at +0.39%, just under the threshold, so it doesn't count.

The accompanying chart plots the S&P 500 monthly on a log scale — where equal steps up the axis mean equal percentage moves — from January 2020 to October 2026. An arrow marks each hot CPI month, orange when the index fell that month and grey when it rose. Only 9 of the 28 arrows are orange, which matches the finding that the same-month effect has weakened. A blue line below tracks US inflation year on year (ECONOMICS:USIRYY), peaking through the hot run of 2021 and 2022.

What does this mean for investors?

Practically: when CPI comes in at 0.4% or more, the next month on the S&P 500 has been a down month about half the time, versus about a third otherwise. That's a meaningful shift in odds. It isn't a crash forecast, and the study doesn't measure how far the market falls when it does fall.

The limits matter too. The analysis ignores what the market expected before the release — a hot print that still beats the forecast can rally. It says nothing about the release day itself, interest rates, or other markets. And it's built on monthly closes and past data.

As the author puts it: observations, not recommendations. But with October's number due on 10 November, the base rate is worth knowing.