Clearing debt is a guaranteed return; investing is not
The claim
Paying down a debt returns its interest rate with certainty, which should be compared to expected investment returns on a risk-adjusted basis, not a raw one.
Why we rate it strong
The core comparison is arithmetic. The judgement about how large an expected-return premium justifies taking risk is where reasonable people differ.
Repaying a debt returns exactly its interest rate. There is no variance, no tax on the return, and no sequence risk. Compare that to a diversified portfolio's roughly 7% nominal expected return with a standard deviation near 17% and a real possibility of losing money over any given decade, and the high-rate case is not close.
The comparison people get wrong is treating an expected return and a guaranteed return as equivalent numbers. A certain 6% and an expected 7% are not one percentage point apart in any meaningful sense — the second carries a distribution wide enough that a full decade of underperformance is unremarkable. This is why our tool leans toward the debt whenever the gap is under roughly two points.
Two things outrank both. An employer match is an immediate return on the contributed amount that no debt rate matches, so it comes first. And a starter cash buffer comes before aggressive repayment, because without one, the next unexpected bill goes straight back onto the card and the effort resets.
Tax changes the arithmetic in both directions. Deductible interest lowers the effective debt rate; taxable gains lower the effective investment return. The calculator handles both.
Where this breaks down
- Liquidity is not symmetric. Money paid into a mortgage is hard to get back out; money in a taxable account is not. That option value is worth something.
- Debts with a fixed low rate become less burdensome as inflation erodes their real value — a real consideration for long-dated fixed-rate mortgages.
- The psychological return on being debt-free is real and shows up in the behavioural data, even where the arithmetic marginally favours investing.
Sources
Follow these rather than taking our word for the summary.
James J. Choi, David Laibson and Brigitte C. Madrian (2011). $100 Bills on the Sidewalk: Suboptimal Investment in 401(k) Plans
The Review of Economics and Statistics, 93(3), 748-763
Finding: A substantial share of employees forgo employer matching contributions they could take with no downside, leaving guaranteed money unclaimed.