Buying a home in the U.S. often involves weighing the trade-offs between a 15-year and 30-year mortgage. With the interest rate staying constant, the first option has higher monthly payments, but the loan is repaid sooner than it is with the second option that offers lower monthly payments.

But home loan borrowers in the U.K., Canada, Australia and most European countries have a wider array of choices: They can break up their loan tenure into smaller chunks of two, three, or five years, and get lower interest rates as their loan size reduces and credit rating improves over time.

A new research paper by Wharton finance professor Lu Liu, titled “The Demand for Long-Term Mortgage Contracts and the Role of Collateral,” focuses on the U.K. housing market to explain the choices in mortgage fixed-rate lengths by mortgage borrowers. She pointed out that the length over which mortgage rates stay fixed is an important dimension of how households choose their mortgage contracts, but that has “not been studied explicitly thus far.” Her paper aims to fill that gap.

Liu explained that the U.K. market is “an ideal laboratory” for the study for three reasons: It offers borrowers an array of mortgage length choices; it is a large mortgage market with relatively risky mortgage loans similar to the U.S.; and it offers the opportunity to study market pricing of credit risk in mortgages. In the U.S. market, the pricing of credit risk is distorted as the government-backed Fannie Mae and Freddie Mac provide protection against defaults. “The U.S. is a big outlier in mortgage structure. It has essentially removed credit risk in the markets for long-term contracts.”

How Beneficial Are Long-term Mortgages?

At first sight, long-term mortgage contracts may seem preferable because they have a fixed interest rate, and thus allow borrowers to protect themselves from future rate spikes, the paper noted. “Locking in rates for longer protects households from the risk of repricing, in particular having to refinance and reprice when aggregate interest rates have risen,” Liu said. “In order to insure against such risks, risk-averse households should prefer a longer-term mortgage contract to the alternative of rolling over two short-term mortgage contracts, provided that they have the same expected cost.”

But in studying the U.K. housing market, Liu found that there is an opposing force that may lead some households to choose less protection against interest rate risk. This has to do with how the decline of credit risk over time affects the credit spreads borrowers pay. She explained how that occurs: As a loan gets repaid over time, the loan-to-value (LTV) ratio decreases as households repay the loan balance and house prices appreciate, the paper noted. This reduces the credit spread that households pay on their mortgage over time. When high-LTV borrowers decide to lock in their current rate, the credit spread will account for a large portion of that rate.

“[30-year mortgages] have had knock-on effects on mobility and housing markets due to mortgage lock-in.” – Lu Liu

As the LTV ratio declines and collateral coverage improves over time, they raise the opportunity cost of longer-term contracts, in particular for high-LTV borrowers, Liu noted. “Locking in current mortgage rates [protects] households against future repricing, but it also locks in the current credit spread, leading households to miss out on credit spread declines over time.”

High-LTV borrowers, or those who opt for low down payments and bigger loans, have to initially pay large credit spreads that can be as high as 220 basis points higher than what a borrower with prime-grade credit would pay. But refinancing with shorter-term contracts allows them to reduce those credit spreads over time. “They’re not locking in to a rate over 30 years; they’re probably locking in at shorter terms of two, three, or five years, and they do it maybe six or seven times,” Liu said. Riskier borrowers with higher LTV ratios hence face a trade-off, as locking in rates while the LTV is high is relatively costly, so they end up choosing shorter-term contracts, meaning they choose less interest-rate protection than less risky borrowers.

“In markets where the credit risk is priced using market prices – without government intervention as in the U.S. — the credit risk is expensive as lenders charge relatively higher rates for that,” Liu said. “If I’m a risky borrower, I face this very difficult trade-off: I want to insure myself like everyone else. But it also means that I’m locking in relatively high rates, with a big credit spread.” That of course does not always make sense for borrowers, she pointed out. “This may help explain why very long-term mortgage contracts with high-LTV mortgage lending are rare across countries.”

Liu said her data, which covered the period from 2013 to 2017, showed that the propensity is lower among riskier borrowers to opt for a 5-year fixed-rate mortgage compared to a 2-year fixed-rate mortgage. The higher the loan-to-value ratio, the lesser was their incentive to choose longer mortgage tenures, her research found. “Borrowers at 95% LTV are less than half as likely to take out a 5-year fixed-rate contract, compared to borrowers at 70% LTV,” the paper stated. The findings help explain the “reduced and heterogeneous demand for long-term mortgage contracts.”

How to Make U.S. Mortgages More Efficient

Liu said the findings in her paper are relevant for mortgage market design. “High-LTV borrowers face a difficult trade-off between their demand to lock in overall interest rate levels, and an expected decline in credit spreads over time,” she said. “Households could benefit substantially from being able to lock in base interest rates, while repricing their credit spreads.”

The findings are important also from both a monetary policy and financial stability perspective, Liu continued. “High-LTV borrowers are more exposed to interest rate risk, which can also cause vulnerabilities in a rising rate environment, since these borrowers may be most affected by mortgage cost increases.”

“There is political resistance to institutional change and borrower resistance to novel mortgage products.” – Lu Liu

The findings of Liu’s research are also timely, given the recent spike in the inflation rate. She noted that the U.S. Federal Reserve has increased interest rates more aggressively than its counterparts in the U.K., Canada, and Australia. All those countries have varying degrees of short-term fixed or variable-rate mortgages. Unlike in those countries, U.S. mortgage borrowers are “relatively shielded from interest rate rises, as the vast majority of households have locked in previous low rates for 30 years,” she noted.

Unintended Consequences of Long-term Mortgage Contracts

But the design of mortgage contracts in the U.S. creates disruptions beyond the housing markets to the broader financial system. “The 30-year fixed-rate mortgages in the U.S. have led to duration mismatch and financial stability risks in the banking sector, as rate rises have reduced the market values of these loans and mortgage-backed securities,” Liu said. She cited the recent collapse of Silicon Valley Bank as a case in point, which was triggered by the fall in the valuation of its bond holdings in a rising interest rate environment. In the U.K., in contrast, banks typically hedge the 2-to 5-year fixed-rate legs of mortgages using swaps, with the remaining part of the contract having a variable rate and thus not causing duration mismatch for the banks.

Long-term contracts have other consequences, too. “The [30-year mortgages] have had knock-on effects on mobility and housing markets due to mortgage lock-in,” Liu continued. Mortgage lock-in occurs in a rising interest rate environment, where homeowners find it a losing proposition to refinance mortgages they had taken out when interest rates were at historical lows. As a result, “people aren’t moving, and the housing market is frozen,” she said.

Liu said policy makers ought to rethink the 30-year fixed-rate mortgage, noting that Harvard economics professor John Y. Campbell had proposed that in a presentation at the Georgia Tech-Atlanta Fed Household Finance Conference in March 2023.

That said, the nature of mortgage systems in different countries is “highly persistent over time,” so any recommendation to radically change them might be far-fetched, Liu noted. “There is political resistance to institutional change and borrower resistance to novel mortgage products,” she added. If the U.S. were to move in the direction of more of a Canadian system that has mortgage rates fixed for five years, she noted, “any implementation of shorter-term fixed-rate contracts would need to take into account the credit risk dimension, which could result in risky households insuring less against interest rate risk.” Such a move has the potential to make monetary policy more effective and the banking system more stable, but further research is needed, she added.