I do not know whether or not it is the norm in Sub-Saharan Africa. However, I do know it was not the norm for Blacks in America until after the passage of welfare. According to Herbert Gutman’s research on the Black Family in Slavery & Freedom covering data from 1750-1925, 85% of Black families were double-headed, which means both mother and father were in the family. Gutman’s research is never quoted because it dispels the myth that slavery is responsible for broken black families. Before you ask, yes Gutman’s research looked at the inner city & rural. More surprisingly was that his study covered unskilled, lower class blacks. Now contrast that too this same group of people who populate these areas today. Single out of wedlock black mothers as single head of households mostly populate the inner city.