Federal Funds Rate: SWSXX.US, being a money market fund primarily investing in short-term US government securities, is highly sensitive to the Federal Reserve's (Fed) monetary policy, particularly the federal funds rate. Increases in the federal funds rate generally lead to higher yields for money market instruments, increasing the attractiveness, and consequently, the price of SWSXX.US.

Treasury Bill Yields: SWSXX.US invests significantly in Treasury bills. The yields on these bills directly influence the fund's return. Higher Treasury bill yields translate to higher returns for the fund and typically a rise in its price. Conversely, declining Treasury bill yields reduce the fund's returns and can lead to a price decrease.

Demand for Short-Term Investments: The overall demand for short-term, low-risk investments impacts SWSXX.US's price. In times of economic uncertainty or market volatility, investors often flock to safe-haven assets like money market funds, increasing demand and potentially driving up the price. Reduced risk aversion can lower demand and the price.

Economic Conditions: Broad economic conditions such as inflation, GDP growth, and unemployment influence interest rate expectations. Strong economic growth may lead to expectations of higher interest rates, which can boost the price of SWSXX.US as its underlying holdings reprice. Weak economic data may have the opposite effect.