Does the S&P Actually Move on IPO Days?

IPO Impact on the S&P 500: A Data-Driven Analysis

Key Concepts:

• IPO (Initial Public Offering): The first sale of stock by a private company to the public.
• S&P 500: A stock market index representing the performance of 500 large-cap companies in the United States.
• Intraday Range: The difference between the highest and lowest price of an asset during a trading day.
• Overnight Gap: The difference between the previous day’s closing price and the current day’s opening price.
• Zeroday Trading: Trading options that expire on the same day they are purchased.
• Positive Drift: The tendency of an asset’s price to increase over time.
• Buy the Rumor, Sell the News: A trading strategy based on the expectation that an asset’s price will rise leading up to a positive event and then fall after the event occurs.

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I. Introduction & Context

The discussion centers around whether significant Initial Public Offerings (IPOs) – particularly a wave expected later in the year involving mature, highly valued companies like OpenAI, Anthropic, and Stripe – can measurably move the S&P 500 index. The analysis specifically investigates if these IPOs, while not immediately added to the S&P 500, create broader market movements ("everything else catches a bid or sell-off"). The goal is to quantify the impact for “zeroday S&P traders” – those focused on same-day expiration options. The speaker notes a perceived lack of prior research specifically addressing this question.

II. Methodology & Data

The study analyzes data from 34 major IPOs between 1995 and 2025. While acknowledging the recent rise of zeroday trading, the dataset predates widespread availability of zeroday S&P options, focusing instead on broader market moves. The analysis compares S&P 500 performance (daily return and intraday range) on the day before, the day of, and the day after an IPO to its baseline behavior.

The IPOs are categorized into three eras:

• .com IPOs: Representing the late 1990s tech boom.
• Post-Bubble IPOs: Following the dot-com bust and the 2008 financial crisis.
• Recent IPOs: Including social media companies (Tesla, Facebook, Twitter, Alibaba) and more recent offerings like ARM, Reddit, ALAB, and Line.

For the most recent IPOs (ARM, Reddit, ALAB, Line), intraday SPX behavior is analyzed at 10-minute granularity, leveraging the availability of zeroday S&P options since March 2023.

III. S&P 500 Baseline Performance

The historical S&P 500 baseline (1995-present) shows a mean daily return (positive drift) of 0.04%. The mean intraday range is 1.27%, with overnight gaps being slightly bullish (positive drift). The last 10 years have exhibited slightly lower volatility than the historical average, though the last 5 years have been more bullish.

IV. Overall IPO Patterns (1995-2025)

Across all 34 IPOs, the following patterns emerged:

• Day Before IPO: Slightly bullish, with a mean daily return of 0.12%.
• Day of IPO: Essentially flat, with a 0.01% downside move.
• Day After IPO: Slightly bearish, with a negative 0.17% average move.
• Week After IPO: A small degree of weakness.

This overall pattern demonstrates a “buy the rumor, sell the news” dynamic in the broader market surrounding IPOs. Overnight gaps leading into major IPOs averaged 0.54%, nearly five times the overall average overnight move (0.01%). Overnight moves following IPOs showed a downside trend, reinforcing the "sell the news" effect. Importantly, these overnight moves were not accompanied by increased volatility.

V. Era-Specific IPO Performance

• .com Era: Showed significant volatility.
• Post-Bubble Era: Demonstrated more moderate moves.
• Social Media/Recent Era: Exhibited the highest volatility, with a mean daily downside move of almost 1.3%.
• Gig Economy, Pandemic, Post-Pandemic & Recent IPOs (ARM, Reddit, ALAB, Line): Consistent volatility compared to average S&P days, again following the “buy the rumor, sell the news” pattern.

VI. Zeroday S&P Data Analysis (March 2023 – Present)

Analysis of intraday data (10-minute granularity) for recent IPOs (ARM, Reddit, ALAB) revealed:

• The S&P 500 typically rallies approximately 0.16% in the first hour of trading on IPO days – four to five times the average daily move.
• This initial rally is largely reversed in the last hour of trading, averaging a negative 0.16% return.
• Overall intraday range was similar or slightly tighter than typical days, but with significant moves around the open and close.

This suggests that while overall intraday range isn’t dramatically increased, the timing of moves is concentrated around the open and close, potentially creating opportunities for zeroday traders.

VII. Case Studies & Examples

The discussion references specific IPOs to illustrate points:

• Tesla, eBay, Pets.com: Cited as examples of major IPOs with significant market impact (both positive and negative).
• ARM, Reddit, ALAB, Line: Used for the granular intraday analysis leveraging zeroday S&P options data.

VIII. Key Takeaways & Recommendations

The primary takeaway is that major IPOs do inject some directional bias into the market, primarily through a “buy the rumor, sell the news” pattern. While intraday ranges are generally similar or slightly tighter than average, there are opportunities for traders, particularly:

• Fading the Move: Capitalizing on the predictable directional shifts before and after the IPO.
• Overnight Trading: Potentially profiting from the bullish overnight move leading into the IPO and the bearish move following it.
• Zeroday S&P Traders: Focusing on the concentrated volatility around the open and close, while being cautious about overall range.

The speaker cautions that the zeroday data set is currently limited (only four major IPOs) and further analysis is needed to draw definitive conclusions. For zeroday S&P traders, treating IPO days as largely “normal” is a reasonable approach, but awareness of the potential for directional moves is advised.

Quote: "The overall takeaway is that you typically see and let's get to the takeaways... you might expect major IPOs to inject volatility in the market, but the data says otherwise." – Speaker.