TL;DR
The empirical question is now being answered in real time, and the answers are mostly unkind to the broad wealth tax: California's billionaire tax is deadlocked against two nullification measures with dueling revenue scores ($100B vs. $40B), the Netherlands formally abandoned taxing unrealized gains, and Norway's own commission recommended retreat — even as Hungary and Sweden push the trend in reverse. The pattern across every jurisdiction this cycle: proposals proliferate, but implementation keeps hitting the same three walls — anticipatory exit, judicial rejection of unrealized-value taxation, and revenue underdelivery.
California's Prop 40: the revenue fight is a fight about the tax base itself
The first-ever popular vote on a wealth tax has become a controlled experiment on one variable — how much billionaire wealth was still sitting in California on January 1, 2026 — and the two sides' answers differ by more than half.
"Six billionaires publicly departed California between the initiative's filing and the January 1, 2026, residency snapshot date, removing $536 billion, or nearly 30 percent of aggregate billionaire wealth, from the tax base." — Hoover Institution press release
"Over the 2019-2025 period, the total amount of California income tax paid by billionaires amounted to only 1.6% of their $1.4 trillion wealth gain, much less than what ordinary Californians pay on their paychecks." — Nobel laureates' letter supporting Proposition 40
The Nobel letter's ~$100B estimate and the Hoover/Rauh estimate of ~$40B minus ~$25B in lost income tax — a negative net present value for the state — rest on the same absence: neither side has administrative data, both are inferring residency from public signals state-level-wealth-tax-proposals-california-2026-initiative+2. Meanwhile the politics run through a dueling-measure endgame: Prop 41 and Prop 42 would each void Prop 40 if they out-poll it, and both currently lead it in PPIC polling (54–43 for Prop 42) state-level-wealth-tax-proposals-california-2026-initiative
+2. Prediction markets agree with the skeptics — Polymarket priced passage around 29% in mid-September.
What to watch: Any post-election adjudication of residency determinations — the first hard data on how many billionaires actually severed California ties before the snapshot date.
The Netherlands quits mark-to-market: the administrability ceiling, demonstrated
The strongest new evidence this cycle comes from a country with one of the world's best tax administrations trying — and failing — to tax unrealized value.
"The Dutch government has abandoned its attempt to tax paper profits... it will instead draft a capital gains tax due only when assets are sold." — IMI Daily
The retreat was a political-economy failure, not a valuation-technology one: Senate stonewalling (36 pages of questions, a 61,000-signature petition), a Supreme Court "Christmas ruling" that had already struck down the old notional wealth tax, and administrative burden. Crucially, the finding's survey of the peer record is blunt: "No European country levies a recurring tax on unrealized capital gains" — the closest analogues are one-time exit taxes in eight of 27 EU states netherlands-box3-unrealized-gains-tax-retreat. The replacement won't reach the statute book before 2029, possibly 2032, at a cost of roughly €3 billion a year in foregone revenue — the price of discovering that recurring unrealized-value taxation has never stuck anywhere in Europe.
What to watch: The Dutch spring 2027 proposal for the realized-gains replacement — whether even the sale-based fallback can pass.
Hungary and the reversal wave: proposals proliferate, capacity doesn't
The European story ran one way for two decades — Germany 1997, Sweden 2007, France's ISF→IFI in 2018 — and this cycle surfaced the first genuine counter-current hungary-wealth-tax-proposal-2026-and-european-reversal-wave. Hungary's new government plans a wealth tax on assets above 1 billion forint (~$3.1 million), with the premier projecting 300–600 billion forint annually — but, as the finding notes, the minister "didn't give details about the Hungarian wealth levy, such as the tax rate or what sort of assets would be subject to it" (Bloomberg via moneycontrol). That revenue range implicitly assumes near-zero avoidance from a country inside the EU's single market, where a Hungarian with $3.1M can relocate to Austria without a visa — the exact condition under which Sweden's and Germany's taxes hemorrhaged their bases. Cutting the other way, Norway's own government-mandated commission has recommended steep cuts to its wealth tax, the first official rollback since the 2021 tightening triggered the billionaire emigration wave hungary-wealth-tax-proposal-2026-and-european-reversal-wave
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What to watch: Hungary's formal legislation — rate, asset scope, official scoring — as the cleanest live test of whether a low-capacity, high-mobility EU state can implement what high-capacity states repealed.
Washington: repeal is outspent six-to-one and still ahead
Washington's 9.9% millionaires' tax faces a three-front endgame — ballot repeal, constitutional litigation, and a re-score — and the money is losing to the polls.
"No on Initiative 645 campaign has raised $7.8 million, far outpacing the $1.3 million for the Vote Yes Repeal the Income Tax campaign." — Spokesman-Review
Despite the 6:1 spending gap, a Cygnal poll found "57% of Washingtonians are opposed to an income tax" — leaning repeal (Clark County Today). The Washington Supreme Court's 4-4 deadlock on ballot language (one justice sick) was procedural only; the real constitutional question — whether income is "property" under Culliton v. Chase — remains open, and a finding for the challengers would make any future Washington wealth tax face an even steeper uniformity bar washington-state-wealth-tax-study-and-constitutional-constraints+1. The administrability datum to watch is the Department of Revenue's revised estimate (~$3.1 billion first-year, ~25,000 households) and the December residency-audit workgroup reports — the first hard read on whether a US sub-federal high-earner levy's base matches its sales pitch.
What to watch: The September 30 Economic and Revenue Forecast Council forecast — the first official test of whether the enacted tax's base matches projections.
The federal pivot: the politicians closest to the fight are conceding the state-level case
The most telling development may be the coalition crack. Gov. Newsom — Prop 40's highest-profile in-state opponent — "pivoted from trying to kill the union's proposal to pitching a national tax that would likely be a key plank of his presidential bid," and Elizabeth Warren echoed that it "should be a federal policy rather than a state one" (Politico). That's an implicit empirical concession: the mobility, valuation, and nullification problems are state-specific, and a citizenship-based federal tax is the one design that structurally solves exit — the US already taxes citizens on worldwide income regardless of residence federal-wealth-tax-proposals-and-constitutional-impact-of-moore. Demand isn't the constraint (53% of US voters say billionaires should pay more); Moore v. United States left the apportionment question for a genuine wealth tax open, which is why every current proposal is drafted as an excise or an income-tax attribute — and why no federal legislation is actually moving.
What to watch: Whether the 2028 primary makes a national billionaire tax the Democratic consensus position — and whether proponents engage the European exit-tax evidence their state-level defeats produced.
What surprised us
- The VC counter-evidence collapses on inspection. The Nobel letter's headline defense against capital flight — California attracting "80% of all new venture capital funding in America" — is directionally corroborated by the LA Times (~90% YTD, $366B), but just two companies, Anthropic ($122B) and OpenAI ($95B), accounted for roughly half of it state-level-wealth-tax-proposals-california-2026-initiative
+2. Both sides are reading two AI mega-rounds as tax-policy evidence.
- Sweden's anticipatory flight is running ahead of any bill. Wealthy Swedes are already relocating assets — or themselves — before re-introduction legislation even exists, replicating the California exit-anticipation effect in the country whose 2007 repeal is the canonical cautionary tale hungary-wealth-tax-proposal-2026-and-european-reversal-wave
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- The Dutch retreat was administrative, not ideological. A coalition of Senate resistance and bureaucratic burden killed it — "very unpopular and more cumbersome and costly to administer for tax authorities" — which matters because it isolates administrability, not politics, as the binding constraint netherlands-box3-unrealized-gains-tax-retreat
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Open threads worth a vote
- How many billionaires actually changed residency before California's Jan 1, 2026 snapshot date? — the $100B-vs-$40B gap rests entirely on this.
- Watch: Hungary publishes wealth tax legislation — rate, asset scope, and official scoring.