from the Wikipedia:
Scientific racism denotes the use of scientific, or ostensibly scientific, findings and methods to support or validate racist attitudes and worldviews. It is based on belief in the existence and significance of racial categories, but extends this into a hierarchy between the races to support political or ideological positions of racial supremacy. Scientific racism can refer to both obsolete and contemporary scientific theories, and includes the use of anthropology (notably physical anthropology), anthropometry, craniometry, and other disciplines in the construction of typologies and the classification of humans into distinct biological races.
The origins of modern scientific racism date from the rise of the science of biology in the 18th and 19th centuries. At that time the development of biology, especially of theories of evolution, undermined then current religious justifications for slavery and for social hierarchy in general. Apologists for inequality looked, therefore, to the new science of biology to support their social views. They first attempted to demonstrate on the basis of anatomical measures like brain size, that blacks and oppressed immigrant groups were "less evolved". By the beginning of the 20th century this approach proved fallacious and scientific racist theorists began using newly developed IQ test as their major argument for intellectual inferiority. The environmentalist school came into prominence after World War II because critical scholarship was demonstrating the IQ test to be invalid as a true measure of mental functioning.
From a genetic perspective, a 1972 study determined that most genetic differences are found within racial groups, not between them. Only 6 percent of such differences were traceable to race.