Should I pay off a 5.99% personal loan aggressively or invest that cash in the S&P 500?

Mathematically, investing in the S&P 500 has historically produced higher returns (about 7%-10% annualized over the long term) than paying off a 5.99% personal loan. Behaviorally, paying off the loan is the safer move because the loan return is guaranteed and the market return is not. The right answer depends on your risk tolerance, your other financial security, and the time horizon you would hold the investment.

The historical math. The S&P 500 has averaged about 10% nominal annual return over the past 50 years, or about 7% after inflation. A 5.99% loan payoff is a guaranteed 5.99% return (the interest you avoid paying). On paper, the 7% historical real return on stocks beats the 5.99% loan, especially after considering that loan interest is paid in after-tax dollars while investment returns can be tax-advantaged in retirement accounts.

The risk-adjusted math. The S&P 500 average is an average; individual years range from -38% (2008) to +38% (1995). Over any 5-year window, returns can be negative (the period from 2000-2005 was -2% annualized). The loan payoff is a guaranteed 5.99% with no volatility. For risk-averse savers, the certainty has value beyond the raw rate.

Tax considerations. Personal loan interest is not tax-deductible. So 5.99% loan interest costs 5.99% in after-tax dollars. Investment returns in a tax-advantaged account (401(k), Roth IRA, HSA) compound tax-free or tax-deferred, meaningfully boosting the effective return. Investment returns in a taxable account are subject to capital gains tax (15%-20% for most investors), reducing the effective return to about 6%-8%.

The match question. If you have an employer 401(k) match available and you are not currently contributing enough to capture it, fund the match first. The match is typically 50%-100% of your contribution up to a cap, which is a 50%-100% guaranteed return. After capturing the match, the loan-vs-invest decision becomes the relevant question.

Emergency fund first. Before either paying down the loan aggressively or investing in stocks, make sure you have 3 months of expenses in cash. Investing in stocks while carrying a low cash buffer creates risk of having to sell stocks during a downturn (when you most need the money) to cover an emergency expense. The forced sale at a loss can wipe out years of returns.

Time horizon matters. If your loan payoff would happen in less than 5 years (say, you have $10,000 left on the loan and could pay it off with one year of focus), pay it off. The 5-year time horizon for stocks is too short to reliably outperform the certainty of debt payoff. If your loan has 10+ years remaining and you are talking about long-term investment in retirement accounts, the math favors investing.

The behavioral question. Some people genuinely cannot tolerate carrying debt and will sleep better knowing the loan is gone. Others find debt-aversion costly and prefer to optimize for return. Both perspectives are valid; the right answer is the one you will stick to consistently. Constantly switching strategies is the worst outcome.

The hybrid approach. Most financial planners recommend paying down high-interest debt (anything above ~7% APR) before investing beyond a 401(k) match, and investing alongside lower-interest debt (below ~5% APR). At 5.99%, you are right at the boundary; either choice is defensible. A 60-40 split (60% extra cash to investment, 40% to loan principal) captures most of the upside of both strategies.