In plain English
Debt is cheaper than equity but it must be repaid on schedule. Capital structure is choosing how much of that cheap-but-rigid money you can safely carry.
The advanced view
Modigliani–Miller sets the baseline: with no taxes, no distress and no information asymmetry, financing is irrelevant because it only re-slices the same cash flows. Every real-world theory is a named violation of an MM assumption — tax shields, distress costs, agency conflicts (trade-off theory), and information asymmetry (pecking order).
Modigliani–Miller is the reference point, not the answer. In a world with no taxes, no bankruptcy costs and no information asymmetry, capital structure is irrelevant: the value of a firm comes from its assets, and slicing the claims differently only reallocates risk. The theorem is useful precisely because it tells you where to look — every real-world capital structure decision is an argument about which MM assumption fails.
Leverage relationships
MM I (no tax): V_L = V_U MM II: r_E = r_A + (r_A − r_D) × D/E With tax: V_L = V_U + t × D (value of the tax shield) β_E = β_A × (1 + (1 − t) × D/E) WACC = E/V × r_E + D/V × r_D × (1 − t)
Add corporate tax and debt gains value because interest is deductible: the tax shield is worth roughly the tax rate times the debt. Add financial distress and the gain reverses at high leverage — customers leave, suppliers tighten terms, managers make short-horizon decisions, and refinancing risk becomes existential. Trade-off theory sets the optimum where the marginal tax shield equals the marginal expected distress cost. Pecking order theory adds the practical observation that managers prefer internal funds, then debt, then equity, because issuing equity signals overvaluation.
Payout policy carries the same information logic. Dividends are sticky, so raising them signals confidence in sustainable cash flow, and cutting them is read as distress. Buybacks are flexible and signal that management thinks the shares are cheap — they also raise EPS mechanically by shrinking the share count, which is why EPS accretion alone is never evidence of value creation. A company returning cash is telling you it has no reinvestment opportunities above its cost of capital; whether that is discipline or decline is the strategic question.
Essential vocabulary
- Tax shield
- The value created by deducting interest from taxable income, approximately the tax rate times debt outstanding.
- Financial distress cost
- Direct and indirect costs of near-bankruptcy: legal fees, lost customers, forced asset sales, management distraction.
- Pecking order
- Financing preference for internal cash, then debt, then equity, driven by information asymmetry.
- Agency cost
- Loss from managers' interests diverging from shareholders'. Debt can reduce it by forcing cash discipline.
- Buyback
- Repurchasing shares to return cash. Reduces share count, raises EPS, and signals perceived undervaluation.
- Hybrid security
- Convertibles, preference shares and mezzanine debt — instruments that sit between pure debt and pure equity.
Strategy connection
Capital structure is strategic capacity. Low leverage buys the option to act during a downturn — to acquire distressed competitors or fund a price war; high leverage buys returns while conditions hold and removes that option. Choose the structure that matches the volatility of the strategy, not the average of the peer group.
Intuition
In a frictionless world capital structure is irrelevant — value comes from assets, not from how they are financed. Everything interesting is a friction: taxes make debt cheap, distress makes it dangerous, and information asymmetry makes issuing equity a signal. The optimum trades the tax shield against the expected cost of distress.
Common pitfalls
- ×Treating a lower WACC from more debt as free value while ignoring rising equity risk.
- ×Forgetting that the tax shield is worthless without taxable profit.
- ×Reading a buyback as value creation when it is only fewer shares.
Worked example — value of the tax shield
Step 1 of 4
- 1Debt 400 permanent, tax rate 25%
Why it works
The irrelevance result works because leverage raises expected equity returns and equity risk in exactly the same proportion: rₑ = rₐ + (D/E)(rₐ − r_d). WACC is unchanged, so value is unchanged. That is precisely why the levered-beta relevering step in a DCF is required — otherwise you double-count the effect of debt.
How it is used — relever a comparable's beta
Step 1 of 5
- 1Comparable: equity beta 1.3, D/E 0.5, tax 25%.
Deeper
Deeper: Modigliani–Miller, then reality
MM with no taxes: capital structure is irrelevant — the pie does not change if you cut it differently, and rising leverage raises ke exactly enough to keep WACC flat. With corporate taxes, the interest shield adds value: V_levered = V_unlevered + t × D.
Reality adds distress costs, agency conflicts and information asymmetry. Trade-off theory sets an optimum where the marginal tax shield equals marginal distress cost. Pecking-order theory says managers prefer internal funds, then debt, then equity, because issuing equity signals overvaluation. In practice companies manage to a rating and a net debt / EBITDA target.
Must know cold
- ✓V_L = V_U + t × D (MM with taxes).
- ✓ke rises with leverage: ke = ku + (ku − kd)(1 − t) D/E.
- ✓Interest cover = EBIT ÷ interest; net debt / EBITDA is the ratings shorthand.
- ✓Distress costs are both direct (fees) and indirect (lost customers, forced sales).
Exercises
Try each one on paper before revealing the worked solution.
Exercise 1
Unlevered value 1,000, tax 25%. The firm adds 400 of permanent debt. What is the levered value and the shield's fragility?
Exercise 2
EBIT 90, interest 15, net debt 250, EBITDA 120. Assess the capacity for another 100 of debt at 6%.