The reason that so many of my leftist friends want to insist that poverty is always just relative is that any objective measure of poverty leads to the conclusion that capitalism is an outrageous success. Only if you raise the sins of envy and covetousness to the status of the moral virtue "fairness" can you make the case that capitalism causes poverty.
The Transformation of Hunger: The Demand for Calories Past and Present
Trevon Logan, Journal of Economic History, June 2009, Pages 388-408
Abstract: According to conventional income measures, American and British industrial workers in the late nineteenth century were two to four times as wealthy as those in developing countries today. Estimated calorie expenditure elasticities of American and British industrial workers based on the 1888 Cost of Living Survey are greater than calorie elasticity estimates for developing countries today, which suggest that yesterday's wealthy workers were hungrier than today's poor. The result is robust to numerous criticisms. The finding implies an extraordinary improvement in nutritional well-being among the poor in the last century that has not been captured by our income estimates.