RDDT Gaps 12.96% on S&P News

RDDT gapped 11.0% at the open and closed up 12.96% at $178.62 after S&P Dow Jones announced it will join the S&P 500 before the open on Tuesday, August 18. It replaces AvalonBay Communities, which is being acquired. The move ranks as the 9th largest single-day gain in RDDT’s 601 sessions as a public company.

The immediate question for traders is straightforward: is buying after an announcement gap up actually profitable?

Index funds have to buy, the demand is fully mechanical, and the shares aren’t in the index yet.

To see if that post-gap edge holds up, we ran the numbers on every eligible S&P 500 addition.

187 additions measured against the market

We took every company added to the S&P 500 since March 2012 and compared each one to SPY over identical timeframes. Relative performance is the metric that matters here. If a stock gains 4% while the broader market gains 4%, it delivered zero alpha.

The data shows that most S&P 500 inclusion gains happen before the stock officially joins the index, with a large chunk of that move occurring before the news becomes official.

Over the 20 trading days leading up to inclusion:

• Median Return: +2.88% (+2.15% alpha vs. SPY)
• Win Rate: 120 out of 187 names beat SPY (~64%)

The announcement week itself accounts for only about a third of those pre-inclusion gains. The rest of the move is priced in early.

After the inclusion date, the edge disappears. Every reading sits within a percent or two of the broader market, and the percentage of stocks outperforming SPY drops to just under half.

Where RDDT Fits

Context matters here. RDDT was down −1.41% over the 20 sessions prior to the news (−5.89% relative to SPY), largely driven by its 21% drop following earnings on July 31.

The August 14 gap to $178.62 simply pushes the stock back to its July 30 close ($178.04). It erased the post-earnings selloff, but it has not broken out beyond where it traded before the drop. With most of the historical inclusion edge concentrated prior to the announcement and performance flattening post-inclusion, chasing the mechanical buying after the pop carries far less relative edge than the headline move suggests.