With no strict criteria for racial classifications, lighter-skinned mulattoes (who obviously were descendants of some Europeans) were easily integrated into the white population. Historically, Europeans took African women as concubines or sexual partners, resulting in mulatto children. Through years of integration and racial assimilation, a white Brazilian population has developed with more historic African ancestry, as well as a black population with European ancestry. In the United States, slavery became a racial caste, and children of slave mothers were considered born into slavery. The efforts to enforce white supremacy after the Civil War and Reconstruction resulted in southern states adopting a one drop rule at the turn of the 20th century, so that people with any known African ancestry were automatically classified as Black, regardless of skin color. At the same time, the United States was receiving millions of European immigrants. In the 21st century, many Black Americans have some degree of European ancestry, while few white Americans have African ancestry.