Thomas Malthus was a British demographer who believed that there was a finite optimum population size in relation to food supply and that an increase in population beyond that point- would lead to a decline in living standards and to ‘war, famine and disease’. His theory was based on two principles.
Food supply, at best, only increases at an arithmetic rate, etc. Malthus considered that this must be so because yields from a given field could not go on increasing forever and the amount of land available is finite.
He considered that population doubles after every 25 years and in 200 years population would increase by 256 times of its original size while subsistence would increase only 9 times of original amount while assuming these rates Malthus intended to highlight the differences between the power of population to increase and the power in the earth to produce subsistence for man.
The ‘positive’ and ‘preventive’ checks which occur in human populations to prevent excessive growth relate to practices affecting mortality and fertility respectively.
According to him Preventive (or negative) checks were methods of limiting population growth and included abstinence from, or a postponement of marriage, which would lower the fertility rate.
Malthus noted a correlation between wheat prices and marriage rates (remember that this was the late 18″‘ century), as food became more expensive, fewer people got married.
Positive checks were ways in which the population would be reduced in size by such events as a famine, disease and war, all of which would increase the mortality rate and reduce life expectancy.