The Myth Ledger · Myth 10 · Too absolute

“Debt is always bad.”

Debt is a contractual claim on uncertain future cash flow. Its quality depends on price, purpose, term, collateral, liquidity, and the borrower’s capacity—not on the word alone.

What survives scrutiny

A revolving balance used to consume beyond income and a fixed-rate mortgage that matches a long-lived housing service share a legal category but not an economic one. Paying debt offers a guaranteed return equal to avoided interest. Borrowing can fund human capital, bridge uneven lifetime income, or diversify exposure across time—but it also creates fixed payments, loss amplification, and forced-sale risk.

Evidence stack

What the literature can—and cannot—say

Tags distinguish measured evidence, economic foundations, model-dependent results, and important limits.

Model result

Human capital changes the balance sheet.²⁹

Stable future labor income can behave somewhat like a large nontradable asset, allowing more financial risk in theory. But income level, volatility, and correlation with markets determine whether that intuition applies.

Model result

Leverage can diversify exposure across time—in a model.²⁸

Ayres and Nalebuff show that constrained young investors may hold little equity relative to lifetime wealth, and model early leverage that spreads market exposure across more years. Financing and path risk are central, not footnotes.

Boundary condition

Households do not borrow in frictionless textbooks.³⁰

Rates, taxes, refinancing, default, complexity, poor diversification, and behavior create costly mistakes. A theoretically positive spread can vanish when the risky asset falls while the payment remains due.

Technical lens

Avoided interest is certain; expected return is not.

Paying down a 20% balance earns a risk-free after-tax return near 20% for that borrower. Comparing it with a risky asset’s average return is invalid without adjusting for risk, tax, fees, and the chance that income disappears at the wrong time.

net spread = after-tax risky return − all-in borrowing cost

Concrete example

The payment survives the forecast error.

A household borrowing at 6% to buy an asset expected to return 8% has a two-point expected spread—not a two-point profit. The asset can fall 40%, the lender still wants 6%, and a job loss can force liquidation. Liquidity and staying power determine whether expected value can be realized.

Decision checklist

A decision rule, not a slogan

Optional checkmarks stay in this browser. JavaScript enables saving; the decision rules below are always readable.

Eliminate revolving and other high-cost debt before reaching for uncertain returns.
Maintain liquidity and insurance before accelerating low-rate debt or investing with leverage.
Match term and rate structure to the asset or service financed; avoid funding long-lived needs with callable short debt.
Use leverage only under a written cap that survives a severe asset drawdown and income interruption without forced selling.

Optional lab: explore the assumptions

The default illustration is readable without JavaScript. Changing its inputs requires JavaScript; outputs are not forecasts.

Counterfactual lab · illustrative

The hurdle the lender actually sets

Compare the guaranteed interest avoided by repayment with an uncertain investment return after a simple tax haircut.

Debt balance$20,000Borrowing APR14.0%Expected risky return8.0%Tax haircut on gains20.0%

Interest avoided

14.0% certain

$2,800 in year one

Expected after tax

6.4%

still uncertain

Expected spread

-7.6%

First-pass decision

Repayment dominates this average-return comparison.

Liquidity can still outrank accelerated repayment when the household lacks an emergency reserve or faces near-term essential spending.

Paying this balance avoids $2,800 of first-year interest. The investment forecast is an average; the loan payment is a contract.

Model boundary · One-year comparison; ignores deductions, compounding, return distribution, basis, fees, default, liquidity value, and debt-specific penalties.

Chapter sources: evidence and limits

Original source numbers are retained across chapters. Each finding travels with its limitation.

28
Diversification Across Time ↗
Ayres, I. & Nalebuff, B. (2013). The Journal of Portfolio Management 39(2), 73–86.
Finding: The authors model modest early-life leverage that spreads equity exposure more evenly across time and can improve modeled retirement outcomes.
Limit: Leverage adds financing, margin-call, behavioral, labor-income, and path risks; the strategy is unsuitable for many households and accounts.
29
Optimal Portfolio Choice for Long-Horizon Investors with Nontradable Labor Income ↗
Viceira, L. M. (2001). The Journal of Finance 56(2), 433–470.
Finding: Labor-income stability and its correlation with markets affect how much financial risk a household can rationally bear.
Limit: Human capital is neither a traded bond nor known with certainty; job loss can coincide with market stress.
30
Household Finance ↗
Campbell, J. Y. (2006). The Journal of Finance 61(4), 1553–1604.
Finding: Households face participation, borrowing, refinancing, diversification, and advice frictions that make textbook optimization difficult.
Limit: Normative models must be translated through actual products, taxes, institutions, and household behavior.