Interest Rate Changes: WMMAX invests in money market instruments; therefore, its yield, and thus its price (Net Asset Value or NAV, which aims to stay at $1), is significantly impacted by prevailing interest rates. Rising rates generally lead to higher yields and potentially slightly higher NAV, while falling rates do the opposite.

Federal Reserve Policy: Actions taken by the Federal Reserve, such as adjusting the federal funds rate or implementing quantitative easing, have a direct effect on short-term interest rates, impacting the returns offered by WMMAX.

Economic Conditions: Overall economic health, including inflation, employment, and GDP growth, influences interest rate expectations. Strong economic data may push interest rates higher, affecting WMMAX positively, and vice versa.

Credit Risk of Holdings: While WMMAX primarily invests in high-quality, short-term debt, there's still some degree of credit risk. Downgrades or defaults of securities held by the fund could slightly impact the NAV.

Supply and Demand: Increased demand for money market funds can slightly increase their price due to increased fund flows, but WMMAX aims to maintain a stable $1 NAV.

Fund Expenses: The fund's expense ratio, which covers management fees and operating costs, directly reduces the yield passed on to investors. Higher expenses can lead to slightly lower relative returns compared to similar funds.