a relative advantage in the struggle for survival and reproduction would be able to pass their characteristics on to further generations. Proponents of Malthusianism were in turn influenced by Darwin's ideas, both schools coming to influence the field of eugenics. Henry Fairfield Osborn Jr. advocated "humane birth selection through humane birth control" in order to avoid a Malthusian catastrophe by eliminating the "unfit". Malthusianism became a less common intellectual tradition as the 19th century advanced, mostly as a result of technological increases, the opening of new territory to agriculture, and increasing international trade. Although a "conservationist" movement in the United States concerned itself with resource depletion and natural protection in the first half of the twentieth century, Desrochers and Hoffbauer write, "It is probably fair to say ... that it was not until the publication of Osborn's and Vogt's books [1948] that a Malthusian revival took hold of a significant segment of the American population". Modern formulation The modern formulation of the Malthusian theory was developed by Quamrul Ashraf and Oded Galor. Their theoretical structure suggests that as long as: (i) higher income has a positive effect on reproductive success, and (ii) land is limited factor of production, then technological progress has only a temporary effect in income per capita. While in the short-run technological progress increases income per capita, resource abundance created by technological progress would enable population growth, and would eventually bring the per capita income back to its original long-run level. The testable prediction of the theory is that during the Malthusian epoch technologically advanced economies were characterized by higher population density, but their level of income per capita was not different from the level in societies that are technologically backward. Preventive vs. positive population controls To manage population growth with respect to food supply, Malthus proposed methods which he described as preventive or positive checks: A preventive check according to Malthus is ways that in which nature may alter population changes. Some primary examples are celibacy and chastity but also contraception, which Malthus condemned as morally indefensible along with infanticide, abortion and adultery. In other words, preventive checks control the population by reducing fertility rates. A positive check is any event or circumstance that shortens the human life span. The primary examples of this are war, plague and famine. However, poor health and economic conditions are also considered instances of positive checks. When these lead to