Social Darwinism is the study and implementation of various theories and societal practices that purport to apply biological concepts of natural selection and survival of the fittest to sociology, economics and politics, and which were largely defined by scholars in Western Europe and North America in the 1870s. Social Darwinism holds that the strong see their wealth and power increase while the weak see their wealth and power decrease. Social Darwinist definitions of the strong and the weak vary, and also differ on the precise mechanisms that reward strength and punish weakness. Many such views stress competition between individuals in laissez-faire capitalism, while others, emphasizing struggle between national or racial groups, support eugenics, racism, imperialism and/or fascism.

Social Darwinism declined in popularity as a purportedly scientific concept following the First World War, and was largely discredited by the end of the Second World War—partially due to its association with Nazism and due to a growing scientific consensus that eugenics and scientific racism were groundless. Later reference to social Darwinism was usually pejorative.

Some groups, including creationists such as William Jennings Bryan, argued that social Darwinism is a logical consequence of Darwinism. Academics such as Steven Pinker have argued that this is a fallacy of appeal to nature as natural selection is a description of a biological phenomenon and does not imply that this phenomenon is morally desirable in human society. While most scholars recognize some historical links between the popularisation of Darwin's theory and forms of social Darwinism, they also maintain that social Darwinism is not a necessary consequence of the principles of biological evolution. Social Darwinism is generally accepted to be a pseudoscience.

Scholars debate the extent to which the various social Darwinist ideologies reflect Charles Darwin's own views on human social and economic issues. His writings have passages that can be interpreted as opposing aggressive individualism, while other passages appear to promote it. Darwin's early evolutionary views and his opposition to slavery ran counter to many of the claims that social Darwinists would eventually make about the mental capabilities of the poor and indigenous peoples in the European colonies. After the publication of On the Origin of Species in 1859, one strand of Darwin's followers, led by Sir John Lubbock, argued that natural selection ceased to have any noticeable effect on humans once organised societies had been formed. However, some scholars argue that Darwin's view gradually changed and came to incorporate views from other theorists such as Herbert Spencer. Spencer published his Lamarckian evolutionary ideas about society before Darwin first published his hypothesis in 1859, and both Spencer and Darwin promoted their own conceptions of moral values. Spencer supported laissez-faire capitalism on the basis of his Lamarckian belief that struggle for survival spurred self-improvement which could be inherited. An important proponent in Germany was Ernst Haeckel, who popularized Darwin's thought and his personal interpretation of it, and used it as well to contribute to a new creed, the monist movement.