Think You Should Never Pay Off Your Low-Interest Mortgage? Think Again.

If you hang out with the optimization nerds, you will eventually be introduced to the gospel of “interest rate arbitrage.”

Here’s the general idea:

If you have a mortgage with an interest rate under 4%, you should stretch it out as long as humanly possible. Paying an extra cent toward your mortgage is mathematically stupid. You must invest every spare dollar into the stock market to capture the spread, otherwise you are throwing away hundreds of thousands of dollars in compound growth.

To be honest, this is mathematically sound, and if you have a long time horizon, extreme discipline as an investor, and a stable financial situation, it often leads to great long-term results.

Historically, a broad index fund will return around 8% to 10% annually. If your mortgage costs you 3% and your investments pay you 9%, keeping the mortgage and investing the difference nets you a massive 6% spread over 30 years.

But, today’s lukewarm take is Your household is not a hedge fund.

Personal finance isn’t just about maximizing basis points on a balance sheet; it’s about maximizing the quality of your actual life. And when we look at the research, optimizing your life for maximum financial leverage ignores the very real psychological cost of carrying debt.

For some (including myself at times if I am being honest), paying down low-interest debt early is sub-optimal. Why would you settle for a guaranteed 3% return (paying off the mortgage) when you could make 9% in the market?

Because debt is inherently a contract that restricts your future freedom.

When you owe a bank hundreds of thousands of dollars, you have to maintain a certain income to service that debt. That means you are less likely to quit a toxic job, start a business, or take a lower-paying job that you actually love.

More importantly, the “optimal” math assumes a perfect future where nothing goes wrong. But what happens during a severe economic recession when your stock portfolio drops by 40%, and your company announces layoffs?

Suddenly, that “cheap” mortgage payment feels like a massive, terrifying anchor around your neck.

To debunk the idea that carrying debt is completely harmless as long as the interest rate is low, we can look at a landmark peer-reviewed study published in the journal Social Science & Medicine titled The High Price of Debt: Household financial debt and its impact on mental and physical health.

The researchers, led by Elizabeth Sweet at Northwestern University, didn’t look at portfolio returns; they looked at the human body. They analyzed the financial data and health outcomes of a massive representative sample of adults to see how debt actually impacts our biology.

Their findings completely dismantle the illusion of stress-free leverage:

The researchers found that higher levels of debt were significantly associated with higher levels of perceived stress and depression. Carrying debt is a chronic, low-grade stressor that constantly hums in the background of your daily life.

It isn’t just in your head.

The study found that individuals with high debt-to-asset ratios had worse self-reported general health and significantly higher blood pressure.

When the optimization nerds (again, largely calling out myself with that term) carry a massive mortgage to invest the difference, they are failing to price in the biological risk.

What good is an extra $100,000 in your retirement account in 30 years if the chronic stress of carrying that debt gives you hypertension in 8 years?

The extreme take demands that you squeeze every penny of potential leverage out of your life, treating your personal finances like a private equity firm that must be optimized at all costs.

But what good is a mathematically perfect portfolio if it makes you miserable and anxious?

This also not to say that it is the ‘right’ choice to pay down debt before investing, it’s not what I do, and it really depends on who you are and what the circumstances of your life are.

After reviewing the research, I’ve kind of separated my goals of investing vs paying down debt: Invest to build wealth, but pay down debt to buy peace of mind.

You don’t have to choose between being a mathematical genius and a debt-free monk.

You can do both.

If you have a low-interest mortgage, you should absolutely be investing a significant portion of your income into the stock market (if you have enough left over money to do that).

That is how you build long-term, generational wealth. But there is absolutely nothing “wrong” about taking a portion of your surplus cash and throwing it at your mortgage principal.

Every time you pay down debt, you are buying a piece of your future freedom back from the bank. You are lowering your biological stress, reducing your monthly risk, and building a financial fortress that can withstand a recession.

Put even simpler, investing raises your financial ceiling while paying down debt raises your floor.

It’s logical that you would want to do both.

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This article is for informational purposes only. It should not be considered Financial or Legal Advice. Not all information will be accurate. Consult a financial professional before making any significant financial decisions.