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"Trickle-down economics" and the "trickle-down theory" are terms in United States politics to refer to the idea that tax breaks or other economic benefits provided by government to businesses and upper income levels will benefit poorer members of society by improving the economy as a whole. The term has been attributed to humorist Will Rogers, who said during the Great Depression that "money was all appropriated for the top in hopes that it would trickle down to the needy." The term is mostly used ironically or as pejorative.
Proponents of tax cuts often claim that savings and investment are essential to the economy, and thus less taxes (for any and all income brackets) need not harm any other income bracket. Economist George Reisman, a proponent of tax cuts, said the following:
Today, "trickle-down economics" is most closely identified with the economic policies known as "Reaganomics" or laissez-faire. David Stockman, who as Reagan's budget director championed these cuts at first but then became skeptical of them, told journalist William Greider that the "supply-side economics" is the trickle-down idea: "It's kind of hard to sell 'trickle down,' so the supply-side formula was the only way to get a tax policy that was really 'trickle down.' Supply-side is 'trickle-down' theory."
Economist Thomas Sowell has written that the actual path of money in a private enterprise economy is quite the opposite of that claimed by people who refer to the trickle-down theory. He noted that money invested in new business ventures is first paid out to employees, suppliers, and contractors. Only some time later, if the business is profitable, does money return to the business owners—but in the absence of a profit motive, which is reduced in the aggregate by a raise in marginal tax rates in the upper tiers, this activity does not occur. Sowell further has made the case that no economist has ever advocated a "trickle-down" theory of economics, which is rather a misnomer attributed to certain economic ideas by political critics who either willfully distort or misunderstand the actual stated goals of their political opponents.
Although the term "trickle down" is mainly political and does not denote a specific economic theory, some economic theories reflect the meaning of this pejorative. Some macro-economic models assume that a certain proportion of each dollar of income will be saved. This is called the marginal propensity to save. Many studies have found that the marginal propensity to save is considerably higher among wealthier people. Policies, including tax cuts, that seek to increase saving are often aimed at the wealthy for this reason. Saving usually means some form of investment, as even money placed in savings accounts is ultimately invested by the banks.
The economist John Kenneth Galbraith noted that "trickle-down economics" had been tried before in the United States in the 1890s under the name "horse and sparrow theory." He wrote, "Mr. David Stockman has said that supply-side economics was merely a cover for the trickle-down approach to economic policy—what an older and less elegant generation called the horse-and-sparrow theory: 'If you feed the horse enough oats, some will pass through to the road for the sparrows.'" Galbraith claimed that the horse and sparrow theory was partly to blame for the Panic of 1896.
Proponents of Keynesian economics and related theories often criticize tax rate cuts for the wealthy as being "trickle down," arguing tax cuts directly targeting those with less income would be more economically stimulative. Keynesians generally argue for broad fiscal policies that are directed across the entire economy, not toward one specific group.
A 2012 study by the Tax Justice Network indicates that wealth of the super-rich does not trickle down to improve the economy, but tends to be amassed and sheltered in tax havens with a negative effect on the tax bases of the home economy.
The Merriam-Webster Dictionary notes that the first known use of trickle-down as an adjective meaning "relating to or working on the principle of trickle-down theory" was in 1944, while the first known use of trickle-down theory was in 1954.
After leaving the Presidency, Lyndon B. Johnson, a Democrat, alleged "Republicans [...] simply don't know how to manage the economy. They're so busy operating the trickle-down theory, giving the richest corporations the biggest break, that the whole thing goes to hell in a handbasket." 
Speaking on the Senate floor in 1992, Sen. Hank Brown said, "Mr. President, the trickle-down theory attributed to the Republican Party has never been articulated by President Reagan and has never been articulated by President Bush and has never been advocated by either one of them. One might argue whether trickle down makes any sense or not. To attribute to people who have advocated the opposite in policies is not only inaccurate but poisons the debate on public issues."
Thomas Sowell claimed that, despite its political prominence, no trickle-down theory has ever existed among economists. In response, many critics referred him to Stockman's remarks to Greider. Sowell replied in his newspaper columns. Stockman himself had not proposed or advocated the alleged theory, so Sowell rejected him as an example of someone who had done so. Additionally, Stockman had not specifically named anyone who, or quoted a source that, advocated the theory although he did claim that the theory was being adhered to by the Reagan administration. Sowell replied that Stockman "was not even among the first thousand people to make that claim" but that "not one of those who made the claim could provide a single quote from anybody who had advocated a 'trickle-down theory.'"