I believe the answer is: - a research paper by a social historian that describes and analyzes African American pop culture in the early twentieth century
- a research paper by a cultural historian that analyzes different jazz styles and how they developed. Economic factors could not be considered as a direct influence of jazz development since there is no proof that jazz is only enjoyed by people on a specific economic power. Because of this, the relevant research should only revolve around the music itself, including the culture and its style development.
A historian is researching how jazz music evolved in the United States in the early twentieth century as a result of the intermingling of African American and European music traditions. I'd say that the two works by other historians that the historian will most likely use to develop a hypothesis on this topic are a research paper by a social historian that describes and analyzes African American pop culture in the early twentieth century and a research paper by a cultural historian that analyzes different jazz styles and how they developed.
