A flight to quality (also called "flight to safety") occurs when investors rapidly shift capital from risky assets (equities, corporate bonds, emerging market debt) into the safety of U.S. Treasury securities. This drives Treasury prices up and yields down, often sharply.
The mechanics:
Demand surge for Treasuries compresses yields, particularly at the long end
Credit spreads widen as investors dump corporate bonds
Equity markets sell off as capital exits risk assets
Liquidity premium on off-the-run Treasuries widens as demand concentrates in the most liquid on-the-run issues
Historical examples:
September 2001: 10-year yield fell from 4.8% to 4.2% in the week following 9/11
October 2008: massive flight to quality during the Lehman bankruptcy and banking crisis
March 2020: initial COVID-19 panic drove a flight to quality (though Treasury market dysfunction briefly disrupted this pattern)
Flight-to-quality episodes typically produce a bull flattener or bull steepener, depending on whether the long end or short end rallies more. They also temporarily restore the negative stock-bond correlation that portfolio diversification depends upon.
The blog post "The End of the Hedge" examines how the reliability of the flight-to-quality mechanism has evolved over different inflation regimes.