Fogel's most famous and controversial work is Time on the Cross (1974), a two-volume quantitative study of American slavery, co-written with Stanley Engerman. In the book, Fogel and Engerman argued that the system of slavery was profitable for slave owners because they organized plantation production "rationally" to maximize their profits. Due to economies of scale, (the so-called "gang system" of labor on cotton plantations), they argued, Southern slave farms were more productive, per unit of labor, than northern farms. The implications of this, Engerman and Fogel contended, is that slavery in the American South was not quickly going away on its own (as it had in some historical instances such as ancient Rome) because, despite its exploitative nature, slavery was immensely profitable and productive for slave owners. This contradicted the argument of earlier Southern historians.