(Thomas) Robert Malthus (1766-1834) was a British economist, professor, and clergyman stationed in a parish at Cambridge, where he was educated.
He is best known in economics for his theory of population, outlined in An Essay on the Principle of Population (1798).
For Malthus, the population usually increases faster than the means of subsistence (available food supply).
According to his theory, whenever population growth slightly exceeds food production, an even higher population growth follows. But if population growth is a great deal higher than food production rate, population growth is limited by famine, pestilence, and war.
Malthus’ ideas challenged the accepted early 19th century view that population growth meant economic growth. Malthusian theory encouraged decreasing the birth rate, which became somewhat popular.
On the downside, his work was often used as a weapon against attempts to improve the lot of the poor.
On the upside, Malthus’ work contributed to the development of contemporary demographics—the statistical study of society. His outlook had an effect on the economist David Ricardo. And Charles Darwin wrote that Malthus’ work influenced the theory of natural selection.
I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long- continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work”.¹
¹ Charles Darwin, autobiography (1876), cited at http://www.ucmp.berkeley.edu/history/malthus.html