Social Darwinism is a complex and controversial topic, a package of ideologies supposedly inspired by biological evolutionism that is of interest to scholars of both the life and the social sciences. In principle it includes any political system inspired by the view that human nature and social activity are driven by our biological nature, especially as defined by the process of evolution. The complexity of the topic derives from the fact that the term social Darwinism has been applied to a number of different (and to some extent incompatible) ideologies. The key feature is supposed to be the influence of Charles Darwin’s theory of evolution by natural selection, in which the “struggle for existence” determines the “survival of the fittest,” thereby ensuring that the species adapts to new conditions—although it is widely assumed that the process also guarantees progress toward higher levels of complexity. The classic image is of the proponents of unrestrained free-enterprise capitalism justifying their policy by appealing to the “survival of the fittest.” But the term has also been applied to justifications of militarism and imperialism (national or racial struggle) and to the eugenics movement’s efforts to replace natural selection with a process of artificial selection by restricting the reproduction of the “unfit.” The term has also been applied to more or less any claim that human nature is fixed by hereditary factors, especially those linked to social class or race. The topic is contentious because social Darwinism is almost always used in a pejorative sense—the opponents of these ideologies use it to define positions they reject, and this becomes particularly sensitive when applied to areas such as Nazi racial policies and the Holocaust. Most forms of social Darwinism are associated with right-wing ideologies, despite the fact that scholars can point to many left-wing writers who were also inspired by Darwin. The problem of interpretation is compounded by the fact that historians of both the biological and the social sciences are involved, bringing very different interpretive frameworks to bear. Scholars interested in the social world tend to equate social Darwinism with any ideology based on the struggle for existence, whether or not there is evidence of inspiration from biological Darwinism. Historians of science may be well aware that the term refers to a much wider range of ideologies than those inspired directly by Darwin, but they do expect the analysis to respect the fact that other biological ideas and, indeed, other evolutionary mechanisms were involved.