The word "eugenics" (from the Greek εὐγενής, for "well-born") was coined by Sir Francis Galton, a cousin of Charles Darwin, to refer to the study and use of selective breeding (of animals or humans) to improve a species over generations. Compared to today's definition, Galton made a confusion between the genetic improvement of the human races by selection of hereditary features considered to be desirable and/or elimination of the traits considered to be undesirable, and the improvement of the individuals by actions related to their living conditions.

However, the initial principle defined by Galton, was directly in connection with the teaching and work of Darwin, himself very influenced by Malthus. According to Darwin, the mechanisms of the natural selection are thwarted by human civilization. One of the objectives of civilization is somehow to help the underprivileged ones, therefore to be opposed to the natural selection responsible for extinction of the weakest. According to eugenicists, the loss of effectiveness could lead to an increasing number of individuals who would have normally been eliminated through natural selection processes. Eugenists thus propose to promote actions to balance effects of natural selection mechanism loss within civilizations. This basic principle inspired numerous and very diverse philosophies, scientific or pseudo-scientific theories and social practices.

In modern usage, it more commonly refers to human selective reproduction with the intent to create children with desirable traits, especially those that best meet some ideal of racial purity, as well as elimination of individuals carrying undesirable traits.

Germany under Adolf Hitler was infamous for its eugenics program, which attempted to maintain a "pure" German race. Among other acts, the Nazis performed extensive, often cruel, experimentation on live human beings to test their genetic theories. During the 1930s and 1940s the Nazi regime sterilized tens of thousands of people who they viewed as mentally unfit. Most of these would later be killed by the Nazis during the war.

The nation that had the second largest Eugenics movement was the United States. Beginning with Connecticut in 1896, many states enacted marriage laws with eugenic criteria, prohibiting anyone who was "epileptic, imbecile or feeble-minded" from marrying (see [1]). Eugenic considerations also lay behind the adoption of incest laws in much of the U.S. Some states also practiced sterilization of "imbeciles" for much of the 20th century. The US Supreme Court ruled in the 1927 Buck vs. Bell Case that the state of Virginia could sterilize those they thought unfit. Over the period when Eugenics was in place over 100,000 individuals were forcibly sterilized. Eugenics also persisted longer in the United States than in any other country with Virginia only halting its sterilizations in 1979.

Almost all non-Catholic western nations adopted some Eugenics legislation, with the notable exception of Britain. Sweden forcibly sterilized 62,000 "unfits" as part of a eugenics program over a forty year period (see [1]). Similar incidents occurred in Canada, Australia, Norway, Finland, Estonia, Switzerland and Iceland for people the government declared to be mentally deficient.

Various authors, notably Stephen Jay Gould, have repeatedly asserted that restrictions on immigration passed in the United States during the 1920s (which were overhauled in 1965) were motivated by the goals of eugenics, in particular a desire to exclude inferior races from the national gene pool. In the early part of the twentieth century the United States and Canada began to receive far higher numbers of southern and eastern European immigrants. Influential eugenicists like Lothrop Stoddard and Harry Laughlin (who was appointed as an expert witness for the House Committee on Immigration and Naturalisation in 1920) presented arguments that these were inferior races who would pollute the national gene pool if their numbers went unrestricted. It is argued that this stirred both Canada and the United States into passing laws creating a hierarchy of nationalities, rating them from the most desirable Anglo-Saxon and Nordic peoples to the Chinese and Japanese immigrants who were almost completly banned from entering the country. However, several people — in particular Franz Samelson, Mark Snyderman, and Richard Herrnstein — have argued, based on their examination of the records of the Congressional debates over immigration policy, that in fact Congress gave virtually no consideration to these factors. Rather, they maintain, the restrictions were motivated primarily by a desire to maintain the US's cultural integrity against the heavy influx of foreigners.

Some who disagree with the idea of eugenics in general contend that eugenics legislation still had benefits; namely, that advocates such as Margaret Sanger (founder of Planned Parenthood of America) found it a useful tool to urge the legalization of contraception.