Economist Robert J. Shapiro's findings arise from his use of a distinctive analytic method, in which he arrayed new Census Bureau data on household incomes into age groups—or "age cohorts"—according to the ages of the household heads. He then traced the households' annual financial progress (or lack of it) through their working lives. The four groups studied were households whose heads were 25 to 29 years old in 1975, 1982, 1991, and 2001.
Shapiro also analyzed each age cohort based on the gender, race and ethnicity, and education of the household heads. He found that across all age cohorts and all demographic groups, median household incomes rose at healthy rates as people aged through the 1980s and 1990s. He also found that from 2002 on, this steady income progress slowed, stopped or reversed in every age cohort and demographic group. He also found that through the entire time period from 1980 to 2013, and across all demographic groups, household incomes rose fastest when the household heads were in their mid-20s to mid-30s, slowed in their 40s, and plateaued in their 50s.