At the time Lee wrote To Kill a Mockingbird, white people had control over the communities they lived in, but many members of the elite class feared that African Americans would make inroads into the white world by marrying and having children with whites. Thus, interracial marriage was outlawed in many states.
Biracial children were referred to as "mulatto," a word derived from "mule," because, like mules, these children were thought to be the offspring of an unnatural union. Ironically, biracial children born to black mothers were not seen as a threat to white superiority, so most people looked the other way when a white man — like Dolphus Raymond in the novel — chose to marry a black woman.
The fear of interracial unions reached its apex in a widely held, unrealistic fear that African American men would rape and impregnate white women as a means of penetrating white society and, worse, white power.
This sort of crime virtually never happened. However, the frenzy that characterized the "rape complex" led to drastic and deadly results: lynching became the primary means of dealing with any accusation of rape of a white woman was pinned on a black man. When the mob comes to lynch Tom Robinson at the jail, Lee alludes to the reality of black men who lived on the receiving end of this treatment.
