In the Danish mortgage market every mortgage is backed by a corresponding bond. Thus, if a home buyer takes out a 500k mortgage at 3% interest, a bond is issued that pays the lender 3% interest on 500k. I’ve written about this system several times before. It has two distinct advantages.

• The correspondence principle means that mortgage banks don’t bear interest rate risk but instead specialize in evaluating credit risk (the risk that the borrower won’t pay). Deep markets rather than banks take on the interest rate risk. This makes the Danish system very stable.
• Mortgages can be pre-paid by buying the corresponding bond at market rates and extinguishing it. If a Danish borrower takes out a 500k mortgage at 3% interest and then rates rise to 6%, for example, the value of that mortgage falls to $358k and the borrower can buy the corresponding bond, deliver it to the bank, and, in this way, extinguish the loan.

In the US, a mortgage can be pre-paid only at a par. As a result, if interest rates rise, home owners don’t want to move because moving would require them giving up a 3% mortgage and replace it with say a 6% mortgage. This is called the lock-in effect. Lock-in can be quite severe. Fonseca and Liu find:

Using individual-level credit record data and variation in the timing of mortgage origination, we show that a 1 percentage point decline in the difference between mortgage rates locked in at origination and current rates reduces moving by 9% overall and 16% between 2022 and 2024, and this relationship is asymmetric. Mortgage lock-in also dampens flows in and out of self-employment and the responsiveness to shocks to nearby employment opportunities that require moving, measured as wage growth within a 50- to 150-mile ring and instrumented with a shift-share instrument.

What about in Denmark? The Danes definitely take advantage of the opportunity to buy-back. Part of this is due to tax advantages but those are just a transfer. More importantly, Danes don’t get locked in. A new paper by Berger, Jeong, Marx, Olesen, and Tourre compares mobility across Denmark and the US:

We study Danish fixed-rate mortgage contracts, which are identical to those in the United States except that borrowers may repurchase their mortgages at market value. Using Danish administrative data, we show that households actively buy back debt when mortgage prices fall below par and that household mobility is largely insensitive when existing mortgage rates are below prevailing market rates — unlike in the United States, where moving rates fall sharply as rates rise. We develop an equilibrium model that explains these patterns and show that introducing a repurchase-at market option into U.S. mortgages substantially reduces interest-rate-induced lock-in with limited effects on equilibrium mortgage rates.

The last point is especially important because you might wonder whether we are assuming a free lunch? After all, if US borrowers lose when they have to pre-pay at par then lenders surely gain. And if lenders gain on pre-payment then they will be willing to lend at lower rates on mortgage initiation. No free lunch, right? The logic is correct but note that the gain to lenders comes mainly from the relatively small set of households that move despite lock-in so the pre-payment bonus to lenders is quite small. Under the author’s calibrated model, mortgage interest rates in the US would rise by only 18 basis points on average if the US moved to a Danish type system.

In other words, there actually is a free or at least a low-priced lunch because lock-in is bad for homeowners and it doesn’t benefit lenders. As a result, moving to a Danish system would create net benefits.