Federal Funds Rate: The primary driver. MGSXX invests heavily in short-term government securities, making its yield highly sensitive to changes in the Federal Reserve's target rate. Higher rates generally lead to higher yields for MGSXX and vice versa.

Short-Term Treasury Yields: MGSXX's portfolio includes US Treasury bills, notes, and other short-term debt. Fluctuations in the yields of these securities directly impact the fund's net asset value and therefore, its price.

Demand for Safe Assets: During periods of economic uncertainty or market volatility, demand for safe haven assets like US government securities increases. This can drive up the price of these securities and consequently, the price of MGSXX.

Fund Flows: Significant inflows into MGSXX can put upward pressure on its price, as the fund manager needs to purchase more assets. Conversely, large outflows can force the fund to sell assets, potentially lowering its price.

Management Fees & Expenses: These costs, deducted from the fund's returns, impact its overall yield and attractiveness to investors, indirectly affecting its price.

Maturity Profile of Holdings: The weighted average maturity of MGSXX's holdings influences its sensitivity to interest rate changes. A longer average maturity makes the fund more sensitive to rate fluctuations.

Credit Risk (albeit minimal): While MGSXX primarily invests in US government securities, there's always a minimal level of credit risk associated with potential defaults or downgrades, which could slightly affect the price.

Inflation Expectations: Rising inflation expectations can lead to higher interest rates, impacting the value of MGSXX's fixed-income holdings and consequently, its price.

Overall Economic Conditions: A strong economy often leads to higher interest rates, affecting MGSXX's yield. Conversely, a weak economy can lead to lower rates and potentially lower yields for the fund.