the population. By 1790 Virginia held 44% (315,000 in a total population of 750,000 the State). It was common in agriculture, with a more massive presence in the South, where climate was more propitious for widescale agricultural activity. By 1790 slavery in the New England States was abolished in Massachusetts, New Hampshire, and Vermont and phased out in Rhode Island and Connecticut. New York introduced gradual emancipation in 1799 (completed in 1827). Pennsylvania abolished slavery during the War for Independence. Robert Fogel and Stanley Engerman, in their 1974 book Time on the Cross, argued that the rate of return of slavery at the market price was close to ten percent, a number close to investment in other assets. The transition from indentured servants to slaves is cited to show that slaves offered greater profits to their owners. A qualified consensus among economic historians and economists is 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", and it is the near-universal consensus among economic historians and economists that slavery was not "a system irrationally kept in existence by plantation owners who failed to perceive or were indifferent to their best economic interests". The relative price of slaves and indentured servants in the antebellum period did decrease. Indentured servants became more costly with the increase in the demand of skilled labor in England. At the same time, slaves were mostly supplied from within the United States and thus language was not a barrier, and the cost of transporting slaves from one state to another was relatively low. However, as in Brazil and Europe, slavery at its end in the United States tended to be concentrated in the poorest regions of the United States, with a qualified consensus among economists and economic historians concluding that the "modern period of the South's economic convergence to the level of the North only began in earnest when the institutional foundations of the southern regional labor market were undermined, largely by federal farm and labor legislation dating from the 1930s." In the decades preceding the Civil War, the black population of the United States experienced a rapid natural increase. Unlike the trans-Saharan slave trade with Africa, the slave population transported by the Atlantic slave trade to the United States was sex-balanced. The slave population