The curriculum in U.S. schools has also contained racism against non-white Americans, including Native Americans, black Americans, Mexican Americans, and Asian Americans. Particularly during the 19th and early 20th centuries, school textbooks and other teaching materials emphasized the biological and social inferiority of black Americans, consistently portraying black people as simple, irresponsible, and oftentimes, in situations of suffering that were implied to be their fault (and not the effects of slavery and other oppression). Black Americans were also depicted as expendable and their suffering as commonplace, as evidenced by a poem about "Ten Little Nigger Boys" dying off one by one that was circulated as a children's counting exercise from 1875 to the mid-1900s. Historian Carter G. Woodson analyzed American curriculum as completely lacking any mention of black Americans' merits in the early 20th century. Based on his observations of the time, he wrote that American students, including black students, who went through U.S. schooling would come out believing that black people had no significant history and had contributed nothing to human civilization, which is not true.