Time-Slice vs. Lifetime Distribution¶

Why It Matters¶

Evaluating snapshot distributions without temporal context can mislead. If inequality is mostly life-cycle variation (young/old, sick/healthy, working/retired), redistribution needs differ from permanent stratification. Permanent income matters more than current income.

The Key Move¶

When evaluating inequality, ask: Is this a snapshot or lifetime view? How much of observed inequality is within-person variation over time vs. between-person permanent differences? Same snapshot inequality has different implications depending on lifetime mobility.

Classic Application¶

Income inequality statistics. Cross-sectional income inequality overstates lifetime inequality if people move through income distribution over lifecycles. 20-year-old interns and 50-year-old executives are same people at different times, not permanent classes.

Surprising Application¶

Academic publication patterns. Junior scholars publish less than senior scholars. Snapshot inequality high. But is this career-stage effect (everyone's trajectory) or permanent stratification (some always publish more)? Determines whether "intervention" is needed or whether patience suffices.

Failure Modes¶

• Treating snapshot inequality as if it were permanent stratification
• Ignoring that high lifetime mobility makes snapshot inequality less problematic
• Assuming all lifecycle variation is natural when some results from unjust access to life-cycle-smoothing
• Using lifetime averaging to hide genuine persistent inequality

Go Deeper¶

Haider & Solon, "Life-Cycle Variation in the Association Between Current and Lifetime Earnings" (2006); Shorrocks, "Income Inequality and Income Mobility" (1978).