Hebous & Mengistu (2024): Efficient Economic Rent Taxation under a Global Minimum Corporate Tax

Summary

"Efficient Economic Rent Taxation under a Global Minimum Corporate Tax" is a working paper by Shafik Hebous and Andualem Mengistu, both of the IMF's Fiscal Affairs Department, dated 6 August 2024, circulated jointly as Oxford University Centre for Business Taxation Working Paper WP 24/10 and IMF Working Paper 2024/057. The authors state the views expressed are their own, not necessarily the IMF's.

The paper asks a question the tax-competition literature on the OECD/G20 "Pillar Two" minimum tax had largely left aside: not how the 15% minimum affects profit shifting and tax competition between governments, but how it affects the domestic choice of profit-tax design — specifically, the two textbook instruments built to tax economic rent without distorting investment, the cash-flow tax and the allowance for corporate equity (ACE). Before Pillar Two, the two designs are formally equivalent: both deliver a zero marginal effective tax rate (METR) on investment earning only the normal return, taxing above-normal profit alone. The paper's central finding is that the minimum tax breaks that equivalence, and breaks it asymmetrically.

The Core Argument / Findings

The mechanism. Pillar Two's top-up tax turns on two quantities computed each year: a "Pillar Two effective rate" (covered tax over covered profit, i.e. "GloBE income"), and a top-up base equal to covered profit minus the substance-based income exclusion (SBIE — 5% of tangible assets plus payroll, after a transition period). If the effective rate falls below 15%, the shortfall applies as a top-up tax on the base. The two rent-tax designs are treated differently by these rules. Immediate expensing (the cash-flow tax's mechanism) is classified under the GloBE rules as a "temporary timing measure": the reduced tax in the expensing year is added back "as if" it were paid, so the Pillar Two effective rate is unchanged and "immediate expensing per se does not trigger a top-up tax" (p. 3). The ACE's notional deduction is treated differently: it is added to profit, which lowers the Pillar Two effective rate and raises the top-up base, so the ACE "can prompt a top-up tax" on its own (p. 3). As the authors put it: "whenever the top-up binds under the R-based cash-flow tax, it must bind under ACE; but it may bind under ACE while not being binding for the R-based cash-flow tax" (p. 4).

Three regions. Comparing the two designs across statutory tax rates produces three regions (p. 3): (i) the minimum tax binds under both designs, with a higher top-up amount and METR under the ACE than under the cash-flow tax; (ii) the minimum tax binds only under the ACE, so the cash-flow tax keeps a zero METR while the ACE does not; and (iii) the statutory rate is high enough that neither design triggers a top-up tax, and the original equivalence is restored.

Where the kink falls differs by design. For the R-based cash-flow tax, the paper derives the threshold exactly: the top-up tax applies whenever the statutory rate is below 15%, and "if τ ≥ 15%, the R-based cash-flow tax retains its efficiency for any investment (METR = 0)" (p. 19) — the threshold in region (iii) is exactly the 15% minimum rate itself. For the ACE, the threshold statutory rate needed to avoid the top-up tax is given by τ = 15%·πᶜ / (πᶜ − 15%·iK), which the paper shows is algebraically above 15% for any positive allowance (p. 24) — and the paper's own numerical illustration shows how far above: at a statutory rate of 25% (well above the 15% minimum), the METR is 0.0% under the R-based cash-flow tax but 13.8% under a non-refundable ACE, or 0.4% under a refundable ACE (p. 30, Figure 8). The conclusion states the asymmetry plainly: "even for high statutory CIT rates, far above 15 percent, the ACE will generate a strictly positive top-up rate. For cash-flow taxation, a statutory rate of 15 percent suffices for preventing a top-up tax" (p. 31).

Policy design implication. Combining an R-based cash-flow tax with a statutory rate of at least 15% keeps the METR at zero on every investment, whether the investing firm is in scope of Pillar Two or not — which the authors say "renders a two-tier system redundant" (p. 31), a straightforwardly more efficient combination than either a sub-15% standard corporate income tax or an ACE. As a fix to the minimum-tax rules themselves, the paper proposes redefining the top-up base as "EBIT minus investment" with carryforward of unused deductions — a cash-flow-like base that "makes the minimum tax compatible with any efficient rent tax design" (p. 5), eliminating the asymmetry at its source rather than requiring countries to work around it.

A second, separate finding: a literature error. Independent of the minimum tax, the paper flags that several official studies (US Congressional Budget Office 2017, US Treasury 2021, OECD 2023, and a 2022 EU Commission study) report negative METRs for ACE systems — a result the authors show is theoretically impossible for a correctly specified rent tax and traces to a modeling error: granting the notional equity allowance against an asset's full initial value rather than its tax-depreciated value in each period. Using their own model's calibration, "for the marginal investment... and τ = 15 percent, the METR is underestimated by 8 percentage points" (p. 21) by that error alone — before Pillar Two enters the picture at all.

Loss offset, separately from the minimum tax. The paper also shows that the cash-flow/ACE equivalence depends on full loss offset (refundable losses, or carry-forward with interest); relaxing that assumption alone, with no minimum tax in the model at all, breaks the equivalence too — the ACE ends up with a lower METR than the cash-flow tax in that scenario "because its NPV of foregone refunds is lower" (p. 31).

Relation to the Georgist Case

This paper is not about land rent, and it does not claim to be: its subject is the mechanics of corporate profit taxation generally — the return to firm-specific and product-market rent, the domain the rent gradient treats as the more contested end of the spectrum, not the clean location-rent case. The paper's interest for Georgist readers is structural rather than substantive: it is a documented case of the same efficiency logic that motivates taxing land rent rather than land improvements — exempt the normal return, tax only the surplus above it — being undermined by an apparently unrelated piece of international-tax architecture. A rule (Pillar Two's substance-based income exclusion and timing treatment) designed for an entirely different purpose — setting a revenue floor against profit shifting — ends up re-taxing the normal return under one efficient-rent-tax design (the ACE) while leaving the other (cash-flow) alone, purely as a side effect of how each design's mechanics interact with a minimum-tax base. The lesson generalizes beyond corporate rent: any rent-tax instrument, land-based or otherwise, that relies on an allowance or notional deduction rather than upfront expensing risks the same kind of interaction if layered under an externally-imposed minimum-tax floor.

Nuances and Limits

• A working paper, not (yet) a peer-reviewed publication. Circulated by IMF Fiscal Affairs Department staff and Oxford CBT; the authors explicitly disclaim that the views are the IMF's own. A revised version has since been listed by the authors as accepted for a peer-reviewed journal under a different title, "Forward-Looking Effective Tax Rates under the Global Minimum Corporate Tax," in IMF Economic Review — the author's own publication list marks it "Forthcoming" (2026) rather than published with a volume and page range; this entry cites the working paper text and results throughout, not the journal version, which was not separately reviewed for this entry.
• A theoretical model, not an empirical test. All results are analytical (closed-form METR/AETR expressions) or numerical illustrations under a stylized parameterization (specific inflation, depreciation, and SBIE assumptions); no country's actual post-Pillar-Two tax filings are examined.
• Rests on Pillar Two rules as understood in mid-2024. The paper notes that, as of May 2024, the GloBE rules did not explicitly specify the treatment of loss refunds or interest-on-carryforward, and the authors adopt the more conservative ("timing measure") reading as their baseline; under the alternative (tax-credit) reading they find the ranking of ACE versus cash-flow taxation can reverse.
• The minimum tax also reintroduces debt bias, a distortion both efficient rent-tax designs are built to eliminate on their own — Pillar Two "tolerates interest deductions... even if the total cost of capital investment is immediately deducted, and it penalizes notional deductions to equity" (p. 31), so a country relying on the cash-flow-tax fix for the top-up-tax problem does not thereby recover full debt-financing neutrality.
• Sections not read at the same depth as the mechanism and results discussed above: the paper's formal Appendix derivations and the standard-CIT baseline model in Section 2 were consulted more lightly than the mechanism (Sections 1 and 4), cash-flow (Section 3), and results/conclusion (Sections 6–7) material this entry draws its claims from.

Bears On

• Research: Hebous, Prihardini & Vernon, "Excess Profit Taxes" — the companion IMF analysis of how to design an ACE/ACC-style rent tax for above-normal corporate profit; that paper's main real-world constraint is cross-border profit shifting, while this paper identifies a second, distinct constraint — the global minimum tax itself — that arises even without any shifting.
• Research: Hebous & Ruf (2017), ACE Systems, Multinational Debt Financing and Investment — the key empirical ACE evaluation, which found ACE regimes did not measurably raise multinationals' real investment even on their own terms; this paper gives a further, purely mechanical reason an ACE can underperform a cash-flow tax once a country is inside the Pillar Two regime.
• Concept: Cash-Flow Tax — this paper is the primary source for the claim that a statutory rate of at least 15% combined with an R-based cash-flow tax keeps the METR at zero even under Pillar Two.
• Concept: Allowance for Corporate Equity — this paper is the primary source for the claim that the ACE, unlike the cash-flow tax, keeps generating a positive top-up tax on the normal return at statutory rates well above 15%.

See Also

• Hebous, Prihardini & Vernon, "Excess Profit Taxes: Historical Perspective and Contemporary Relevance"
• Hebous & Ruf (2017): ACE Systems, Multinational Debt Financing and Investment
• Cash-Flow Tax
• Allowance for Corporate Equity
• Economic Rent

Sources

1. Shafik Hebous & Andualem Mengistu (2024), "Efficient Economic Rent Taxation under a Global Minimum Corporate Tax," Oxford University Centre for Business Taxation Working Paper WP 24/10 (also circulated as IMF Working Paper 2024/057; SSRN 4868726), 6 August 2024. PDF — used for all findings, propositions, quotations, and page-cited numbers above; an IMF working paper by Fiscal Affairs Department staff (Tier 1, official/standing body per EDITORIAL §4c), cited as a working paper (B-claim) rather than as peer-reviewed. The mechanism, three-region result, and results sections (roughly pp. 1–31 of 63) were read closely; the Appendix formal derivations were consulted more lightly.
2. Shafik Hebous, "Research" (author's website), listing "Forward-Looking Effective Tax Rates under the Global Minimum Corporate Tax" (with Andualem Mengistu), IMF Economic Review, 2026, Forthcoming, under Peer-Reviewed Articles, with no corresponding working-paper listing under that title. shafikhebous.com — used only to support the "forthcoming journal version" note in Nuances and Limits (B-claim; the journal article's own text was not read, so no claim in this entry beyond the working paper rests on it).