Modern humans are genetically very similar. Research from the Human Genome Project, completed in 2003, found that people share about 99.9% of their DNA, and there is more variation within so-called racial groups than between them. Racial categories were built by European scientists and colonial governments to justify slavery and conquest.
US law made race real in its effects. The Naturalization Act of 1790 limited citizenship to "free white persons." Southern states later adopted the one-drop rule, which classified anyone with any known African ancestry as Black. The census has changed its racial categories nearly every decade, a sign of how political the lines are.
Race still shapes life outcomes because laws and institutions were organized around it for centuries. Federal agencies use race data to enforce civil rights law and track gaps in health, housing and wealth. In 2024, the federal government revised its standards to combine race and ethnicity into one question and add a Middle Eastern or North African category.