Reddit Joins the S&P 500 Tuesday. Here's What History Says Could Come Next -- and What Investors Should Know.

RDDT joins the S&P 500 on Tuesday, August 18, replacing AVB in the index. The stock jumped 13% on the announcement, but historical data shows index inclusion gains have eroded sharply—from 7.4% in the 1990s to less than 1% in the 2010s, according to research cited by The Motley Fool.

The social media platform now trades at 40x earnings despite mounting headwinds. Google's AI-generated search answers threaten to cut referral traffic to Reddit, a core driver of user growth and ad revenue. While the company has delivered strong revenue expansion and recently turned profitable, the valuation premium leaves minimal margin for error.

EQR, another apartment REIT, remains in the index as AVB exits to make room for RDDT. TSLA, already a member, provides a reference point for tech names carrying elevated multiples entering the benchmark.

The fade in index-inclusion alpha reflects growing index arbitrage sophistication and the rise of passive flows that front-run official additions. Reddit's fundamentals must sustain its 40x multiple without the tailwind earlier generations of S&P entrants enjoyed.