On September 20, 2023, the S&P 500 dropped 1.6% within hours of the Federal Open Market Committee's statement, and the yield on the 10-year Treasury climbed to 4.49%. The move was expected. What repriced was the relationship between every major asset class and the assumption that had underpinned portfolio construction for most of the prior decade.

A quarter-point hike is 25 basis points, or 0.25%. That number sounds small until you see what it does to the math behind valuations. Stocks are priced as the present value of future cash flows, discounted back to today using a rate that reflects the risk-free return available elsewhere. When the Federal Reserve raises rates, Treasury yields rise, and that discount rate rises with them. A company expected to generate $100 million in profit five years from now is worth less today if the alternative, lending to the U.S. government, now pays 4.5% instead of 4.25%. The 25 basis points didn't destroy the profit. It changed what someone will pay for it now.

Why growth stocks move first

The effect is not evenly distributed. Growth stocks, companies whose value depends overwhelmingly on earnings expected years in the future, are the most sensitive to changes in the discount rate because the bulk of their value sits far out on the timeline. A biotech firm with no current profit but a Phase III drug expected to generate revenue in 2028 has almost all its value in that distant payoff. Raise the discount rate by 25 basis points and the present value of 2028 revenue falls by several percentage points. A utility that pays dividends today from steady cash flow barely moves, because most of its value is in near-term cash that doesn't get discounted as steeply.

This is why technology stocks fell harder than the broader index in September 2023. The sector had spent a decade priced for a world where Treasury yields hovered near 1% and there was no alternative. TINA, There Is No Alternative to stocks, was the acronym that ran portfolios. When the Fed pushed the federal funds rate into the 5.25%, 5.50% range, TINA died. Bonds became competition.

The bond math that runs backward

Bond prices and yields move in opposite directions, and this is where the repricing becomes visible in real time. A bond issued last year paying 3.5% annually becomes less attractive the moment new bonds are issued paying 4%. To compete, the old bond's price must fall until its fixed coupon payment represents a yield closer to 4%. The holder of that bond took a loss when the Fed moved rates and the market recalculated what the bond is worth.

For a decade, rising bond prices had been the norm because rates kept falling. Investors who bought bonds in 2015 watched their value climb as yields dropped. The 2022, 2023 tightening cycle reversed that entirely. Bond funds posted losses. Retirees who had been told bonds were the safe part of the portfolio saw account balances drop. The safety was about predictable income and low correlation to stocks. When rates rise, bonds lose value. The income still pays, but the principal moves.

What changed beyond the numbers

The deeper repricing was psychological. Markets had spent years interpreting strong economic data, solid job numbers, rising consumer spending, as bullish. More growth meant higher corporate earnings. After the Fed made clear that inflation was the priority, that logic flipped. Strong job numbers in August 2023 sent stocks down because they signaled the economy could withstand more rate hikes. Good news became bad news. The model investors had been running for a decade no longer matched the environment.

The quarter-point hike in September was one of eleven rate increases between March 2022 and July 2023. Individually, 25 basis points is a policy tweak. Cumulatively, the moves redrew the math behind every portfolio allocation decision made since 2015.