Why the MM Theorem Is Not a Special Case of the Coase Theorem

Key Takeaways

• MM assumes costless contracting; Coase assumes zero transaction costs.
• Coase highlights opportunity‑cost pressure; MM shows financial indeterminacy.
• MM requires strong‑form market efficiency, no taxes, fixed investment.
• Coase’s insight underpins agency theory and corporate governance mechanisms.
• Both theorems shape modern corporate finance research and policy.

Summary

A new paper argues that the Modigliani‑Miller (MM) theorem and the Coase theorem are conceptually distinct and neither is a special case of the other. The authors highlight that MM relies on strong‑form market efficiency, no taxes and fixed investment, while Coase assumes costless contracting and emphasizes opportunity‑cost pressure. They show that MM yields indeterminate financial choices, whereas Coase delivers a determinate optimal allocation. The analysis reframes how each theorem informs corporate finance, agency theory and governance research.

Pulse Analysis

The Modigliani‑Miller (MM) theorem and the Coase theorem are often grouped together because each delivers an ‘irrelevance’ result under idealized conditions. Yet the underlying assumptions diverge sharply. MM rests on strong‑form market efficiency, frictionless trading, no taxes, and a fixed investment plan, allowing any debt‑equity mix to leave firm value unchanged. By contrast, Coase assumes costless contracting and focuses on the allocation of property rights, showing that when transaction costs vanish the initial rights distribution does not affect the efficient outcome. This distinction makes the two theorems conceptually independent rather than nested.

The practical lesson from MM is that investment decisions, not financing choices, drive value—a point that sparked the agency‑cost literature of Jensen, Meckling and Myers. Coase’s contribution, however, is the recognition of opportunity‑cost pressure: parties will continually seek arrangements that eliminate allocative inefficiencies, even when contracts are costly. This pressure fuels governance mechanisms such as shareholder activism, board monitoring, and incentive‑based compensation. By separating the theorems, scholars can better attribute why certain corporate finance phenomena arise from market imperfections versus the strategic bargaining environment described by Coase.

Understanding that MM is not a special case of Coase reshapes how academics and practitioners model corporate behavior. It clarifies that MM’s indeterminacy applies only under a narrow set of frictionless market conditions, while Coase’s framework remains relevant across real‑world settings with costly information and enforcement. Consequently, policy discussions about capital‑structure regulation, tax reform, or corporate governance can draw on the appropriate theorem without conflating their premises. Future research is likely to explore hybrid models that embed Coasian opportunity‑cost dynamics into otherwise MM‑style valuation, offering richer predictions for firms operating in imperfect markets.