Noting that while food production tends to increase arithmetically, population tends to increase naturally at a geometric rate, Malthus argued that it is no surprise that people thus choose to reduce population growth. People can increase food production, Malthus thought, only by slow, difficult methods such as reclaiming unused land or intensive farming; but they can check population growth more effectively by marrying late, using contraceptives, emigrating, or, in more extreme circumstances, resorting to reduced, health care tolerating vicious social diseases or impoverished living conditions, warfare, or even infanticide. Malthus was fascinated not with the inevitability of human demise, but with why humans do not die off in the face of such overwhelming odds. As an economist, he studied responses to incentives.
The demographic transition model seeks to explain the transformation of countries from having high birth and death rates to low birth and death rates. The model is based on the change in crude birth rate (CBR) and crude death rate (CDR) over time.
E. G. Ravenstein, a Fellow of the Royal Geographic Society, outlined a series of "laws of migration" that attempted to explain and predict migration patterns both within and between nations.
The gravity model, as social scientists refer to the modified law of gravitation, takes into account the population size of two places and their distance. Since