The authors define "Absolute Income Mobility" as the fraction of people who earn more than their parents. They observe that income mobility, under this definit

The authors define "Absolute Income Mobility" as the fraction of people who earn more than their parents. They observe that income mobility, under this definition, has been declining since 1940. They seem to regard this as a problem, which reflects a diminishing level of opportunity for each subsequent generation. The data they present are interesting and worth further study, but I do not think I agree with their subjective assessment (that this is a problem to solve). On seeing the data, my first hypothesis is that a major driver here is the fraction of a birth cohort that is more educated than their parents. People born in the 1940s and 1950s are much more likely to have more education (and more income) than their parents, compared to people born in the 70s and 80s. But if you're born later then it is more likely that your parents are "fully educated", in which case why should you expect to be able to out-earn them?