U.S. equities and stock exchange traded funds have made a huge rally this month after the recent bout of volatility. However, ETF investors may have to pare expectations going toward the end of the year.

The S&P 500 Index and SPDR S&P 500 ETF (NYSEArca: SPY) gained 8.1% month-to-date ended October 23, or about eight times the amount recorded in a typical October since 1945, Sam Stovall, U.S. Equity Strategist for S&P Capital IQ, wrote in a research note.

While markets have done the best in the fourth quarter in all years since 1945, history shows than an advance in excess of 7%, which has only happened five times since World War II, typically resulted in a gain of just 1.9% over November and December of the year, with the S&P 500 rising three of five times.

“In other words, a strong October tend to suppress the surge associated with a Santa Claus rally,” Stovall warned. “If history is any guide, for it’s never gospel, a strong performance in October typically has resulted in a sub-standard performance for the final two months of the year.”

Since 1945, the S&P 500 rose in price 77% of the time during the final two months of the year, with an average price gain of 3.0%. Each minimum increase in the price advance over October diminished the average price gain and frequency of advance for the rest of the year. In contrast, the best rest of the year returns typically occurred after an October decline, which occurred 27 times with an average price increase during the rest of the year of 3.4%. Consequently, investors who are looking for a Santa Claus rally may be wishing for the markets to tank over the next couple of sessions before the month ends.