Castle Securities sounds the alarm: The "September curse" combined with cheap options has sharply deteriorated the short-term risk-reward outlook for U.S. stocks.

September is typically the worst-performing month for U.S. stocks, with the average monthly return on the S&P 500 Index at its lowest level of the year. Currently, options prices are also at their cheapest levels of the yearcombined, this makes the risk-reward ratio for buying downside protection quite attractive. This is the core viewpoint from Scott Rubner, Head of Equity and Equity Derivatives Strategy at Citadel Securities, in a report to clients on Monday (August 31). Rubner pointed out that the bullish pattern that drove the S&P 500 index to a historic high in August is now changing. This does not indicate a shift in our long-term bullish stance on stocks, but rather a change in the short-term risk-reward dynamics, Rubner wrote in the report. He cited the earnings calendar, outlook for stock buybacks, seasonal factors, and retail trading patterns as reasons for caution. Overall, these factors change the short-term asymmetry. The catalysts for upward movement are becoming less clear, while catalysts for downward movement are increasing. Consolidation and a Volatility Low after Historic Highs In August this year, the S&P 500 index briefly reached a historic high of 7816.70 points, following a nearly 7% increase from the end of July to the first week of August. However, the benchmark index subsequently entered a period of consolidation or even slight decline. At the same time, the Chicago Board Options Exchange Volatility Index (VIX) fell to 14.1 last week, marking its lowest level of the year. Mandy Xu, Head of Derivatives Market Analysis at Cboe, noted in a report early Monday that stronger-than-expected earnings from tech stocksespecially Nvidias (NVDA) performance last weekhelped compress the volatility risk premium for tech stocks, alleviating concerns surrounding AI trading. Cboe data indicates that the individual stock volatility, which was previously at historic highs, has also retreated compared to the index volatility. The VIXEQ index, which measures the implied volatility of the top 50 S&P 500 constituents, has seen a significant narrowing of spread; the spread tracking Nasdaq 100 volatility between QQQ and SPY has dropped from a historic high in June to a year-to-date low at the 20th percentile. Downside Protection is Cheap, but Retail Buying is Weak Rubner believes that this is why the current cost of buying downside protection is relatively low. However, the challenges in September are not only based on historical statistics. Rubner pointed out that September is also the month with the weakest buying from retail traders, according to the retail trading activity tracked by Citadel Securities. Since 2019, the average net buying by retail investors on down days for the S&P 500 has only been about half the annual average. In addition, Rubner anticipates that corporate buybacks will also slow downstarting around September 12, publicly traded companies will enter a blackout period for buybacks, and the related restrictions will gradually tighten. A Period of Increased Macroeconomic Events Arrives, Yet Protection Premiums are Low Investors are entering a period of more frequent macroeconomic events, yet paying relatively low premiums for protection, Rubner stated. Just in mid-August, Rubner had assessed that a technical reset in U.S. stocks was largely complete, with systematic strategies likely to re-leverage. At that time, he noted that systematic capital, retail investors, passive ETFs, and corporate buybacks were forming a mutually reinforcing buying pattern. However, just half a month later, with the arrival of September, historically the weakest month, his short-term stance has clearly shifted to being cautiousthe fading upward catalysts alongside accumulating downside risks and low protection costs create a fundamental short-term contradiction in the current market. For investors, after enjoying the calm and upward momentum of summer, Septembers market may require extra vigilance.