multiplied nearly fourfold between 1810 and 1860, despite the passage of the Act Prohibiting Importation of Slaves signed into law by President Thomas Jefferson in 1807 banning the international slave trade. Thus, it is also the universal consensus among modern economic historians and economists that slavery in the United States was not "economically moribund on the eve of the Civil War". In the 2010s, several historians, among them Edward E. Baptist, Sven Beckert, Walter Johnson and Calvin Schermerhorn, have posited that slavery was integral in the development of American capitalism. Other economic historians have rejected that thesis. Efficiency of slaves Scholars disagree on how to quantify the efficiency of slavery. In Time on the Cross Fogel and Engerman equate efficiency to total factor productivity (TFP), the output per average unit of input on a farm. Using this measurement, Southern farms that enslaved black people using the gang system were 35% more efficient than Northern farms, which used free labor. Under the gang system, groups of slaves perform synchronized tasks under the constant vigilance of an overseer. Each group was like a part of a machine. If perceived to be working below his capacity, a slave could be punished. Fogel argues that this kind of negative enforcement was not frequent and that slaves and free laborers had a similar quality of life; however, there is controversy on this last point. A critique of Fogel and Engerman's view was published by Paul A. David in 1976. In 1995, a random survey of 178 members of the Economic History Association sought to study the views of economists and economic historians on the debate. The study found that 72 percent of economists and 65 percent of economic historians would generally agree that "Slave agriculture was efficient compared with free agriculture. Economies of scale, effective management, and intensive utilization of labor and capital made southern slave agriculture considerably more efficient than nonslave southern farming." 48 percent of the economists agreed without provisos, while 24 percent agreed when provisos were included in the statement. On the other hand, 58 percent of economic historians and 42 percent of economists disagreed with Fogel and Engerman's "proposition that the material (not psychological) conditions of the lives of slaves compared favorably with those of free industrial workers in the decades before the Civil War". Prices of slaves The U.S. has a capitalist economy so the price of slaves was determine