So one theory is the Malthusian theory which was first discussed by Thomas Malthus in 1798 when Malthus became concerned about rapid population growth. He came up with a theory focusing on food availability and how this interacts with populations. His main point is that population is ultimately determined by food availability. If the food is there, populations will continue to grow. However, food availability increases at a slower, arithmetic rate than human populations which grow exponentially at a geometric rate. Population will continue to grow unless put in check. Malthus proposed two types of check. First is the preventative check. This brings down the birth rate and is applied by man. An example is the Chinese One Child policy. Secondly is the positive check which increases birth rate and is applied by nature. An example is famine and disease. A second theory is that of Boserup which countered Malthus' theory on food supply. A key phrase of hers was that "necessity is the mother of invention". This means that technology can prevent populations from collapsing and therefore increase the amount of food available. This is a more optimistic view than Malthus as where food supply is short and Malthus would emphasise that a 'check' should be put into place, Boserup argues that we should just upgrade the productivity of our food supply instead. An example of Boserup's theory in work is the Green Revolution in India.