The economy of the United States is the world's largest national economy. Its nominal GDP was estimated to be $14.3 trillion in 2009, approximately a quarter of nominal global GDP. Its GDP at purchasing power parity was also the largest in the world, approximately a fifth of global GDP at purchasing power parity. The U.S. economy also maintains a very high level of output per capita. In 2009, it was estimated to have a per capita GDP (PPP) of $46,381, the 6th highest in the world.
Historically, the U.S. economy has maintained a stable overall GDP growth rate, a low unemployment rate, and high levels of research and capital investment funded by both national and, because of decreasing saving rates, increasingly by foreign investors. It has been the world's largest national economy since 1944 and remains the world's largest manufacturer, representing 19% of the world's manufacturing output. In 2009, consumer spending, coupled with government health care spending constituted 70% of the American economy. About 30% of the entire world's millionaire population reside in the United States (in 2009). Furthermore, 40% of the world's billionaires are American. The US is also home to the world's largest stock exchange, the New York Stock Exchange. It also boasts the world's largest gold reserves and the world's largest gold depository, the New York Federal Reserve Bank. The United States is also home to 139 of the world's 500 largest companies, which is almost twice that of any other country. A large contributor to the country's success has also been a very strong and stable currency. The US dollar holds about 60% of world reserves, as compared to its top competitor, the euro, which controls about 24%.
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