A sovereign wealth fund (SWF) is a state-owned investment fund investing in real and financial assets such as stocks, bonds, real estate, precious metals, or in alternative investments such as private equity fund or hedge funds. Sovereign wealth funds invest globally. Most SWFs are funded by revenues from commodity exports or from foreign-exchange reserves held by the central bank. By historic convention, the United States' Social Security Trust Fund, with $2.8 trillion of assets in 2014, is not considered a sovereign wealth fund.