Estimates for 1820 to 1970 are based on the estimates on the share of people living in extreme poverty from Bourguignon and Morrisson. 2002. “Inequality Among World Citizens: 1820-1992 .” American Economic Review, 92 (4): 727-744. The series is poverty headcount (1.90$ per day) as far back as possible (1981) from the World Bank, and all observations before 1980 are from Bourguignon and Morrison. 2002. â€œShare of the World Population Living in Extreme Poverty.â€