Marcelo Medeiros and Pedro Souza (two excellent researches) give an answer to this question in a recent study:
We review the literature about the rich, the affluent and the top incomes focusing in two issues: identification and measurement, and the analysis of the determinants of richness. The review discusses data sources, indicators, populations and units of analysis used for the identification of the rich, approaches used to construct affluence lines and measures of richness. It also surveys empirical results about the composition of the incomes of the rich and the role of direct determinants of richness, such as individual characteristics, the State and the structure of production. We cover literature since the early twentieth century but give special attention to the research conducted after the 2000s.