Inequality measurement based on static snapshots (comparing top 1% in 1980 vs 2010) is fundamentally flawed because the people in those categories change: a more accurate measure of inequality is the fraction of the population that spends at least one year in the top 1% (approximately 15% of Americans) or top 10% (approximately 60% of Americans), which reveals Americans have far more mobility than a static comparison suggests, contrasting with Europe's flatter structure with lower mobility.