In California, a study by the National Bureau of Economic Research estimates that Proposition 40, a temporary 5% tax on billionaire wealth, would generate $30 billion in revenue but result in net losses of $38.9 billion after accounting for tax-related departures. Economists Benjamin Jaros and Joshua Rauh estimate that nearly 30% of the affected tax base had already left the state before January 1, 2026. Their findings are disputed by Emmanuel Saez and by California’s administration, which projects tens of billions of dollars in temporary revenue and annual income tax losses of less than $1 billion. These conclusions fuel France’s debate over the Zucman tax, although direct comparison is limited by differences in mobility, tax residency and tax-base rules between California and France.
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