The United States America has a capitalist mixed economy, which is fueled by abundant natural resources, a well-developed infrastructure, and high productivity. The economy of the United States is the world's largest national economy. Its gross domestic product (GDP) was estimated as $13.8 trillion in 2007. The U.S. economy maintains a high level of output per person (GDP per capita, $46,000 in 2007, ranked within the top ten highest by most sources). The U.S. economy has maintained a stable overall GDP growth rate, a low unemployment rate, and high levels of research and capital investment. Major economic concerns in the U.S. include national debt, external debt, entitlement liabilities for retiring baby boomers who have already begun entering the Social Security system, corporate debt, mortgage debt, a low savings rate, and a large current account deficit. In 2008, seventy-two percent of the economic activity in the U.S. came from consumers.The economic history of the United States has its roots in European settlements in the 16th, 17th, and 18th centuries. The American colonies progressed from marginally successful colonial economies to a small, independent farming economy, which in 1776 became the United States of America. In 230 years the United States grew to a huge, integrated, industrialized economy that makes up over a quarter of the world economy. The main causes were a large unified market, a supportive political-legal system, vast areas of highly productive farmlands, vast natural resources (especially timber, coal and oil), and an entrepreneurial spirit and commitment to investing in material and human capital. The economy has maintained high wages, attracting immigrants by the millions from all over the world.