The folly of debt sounds like the opening of a sermon, and taken too broadly it is simply wrong. Plenty of debt is sensible: a mortgage that buys a home, a loan that funds an education or a business, cheap borrowing against something likely to grow in value. Used that way, debt is a tool, not a vice. The folly the phrase really points at is narrower and more specific: the high-cost consumer debt that people take on against their own long-term interest, on credit cards, buy-now-pay-later plans, and payday loans. The genuinely interesting question is not whether that kind of debt is unwise, which it usually is, but why otherwise sensible people fall into it so reliably. The answer lies in two well-documented quirks of the human mind, one about time and one about arithmetic.
Why debt is so tempting
The first quirk is that we are systematically impatient in a particular, lopsided way. Behavioural economists call it present bias: people apply a very steep discount to the near future, valuing an immediate reward far more heavily than a larger cost or benefit further off, which produces choices our calmer, longer-term self would not endorse.
Consumer debt is almost perfectly designed to exploit this. Borrowing to buy something now delivers the pleasure immediately and pushes the pain of paying into a future that barely registers in the present moment’s calculation. The card, the instalment plan, and the loan all separate the reward from the cost in exactly the way present bias finds hardest to resist, letting you enjoy the thing today while a distant, discounted version of you handles the bill. This is why debt is rarely a failure of character or values. It is a structural feature of how humans weigh time: the present self that wants the thing simply outvotes the future self that will have to repay it, and the future self does not get a say until it is too late.
Why we underestimate what it costs
The second quirk turns temptation into genuine miscalculation, because we are also bad at the maths of compounding. Research by Craig McKenzie and Michael Liersch on what is known as exponential growth bias found that people intuitively treat exponential growth as if it were roughly linear, and so badly underestimate how much a compounding quantity grows over time.
Their demonstrations focused on savings, where the bias makes people underestimate how much a nest egg will grow, but it runs in exactly the same direction for debt, and there it is dangerous rather than merely disappointing. A balance carried on a high interest rate does not grow in a gentle straight line; it compounds, and because our intuition quietly flattens that curve, we systematically underestimate how expensive borrowing becomes. The true cost of carrying a credit card balance or rolling over a high-rate loan is far larger than it feels, and it feels manageable precisely because the mind cannot, without deliberately doing the sums, sense the shape of exponential growth. So the folly of debt is not only impatience. It is that we take it on partly because we literally cannot feel how much it will cost us, and by the time the compounding becomes visible, it is working against us at full strength.
Telling folly from sense
Put the two together and the real distinction comes into focus, which is not between debt and no debt but between two very different kinds of borrowing. Debt used to acquire something that grows or earns, at a manageable rate, can be perfectly rational. High-interest revolving debt taken on for consumption is the folly, because it is the kind that present bias makes irresistible and exponential growth bias makes invisible, and it carries a further cost beyond the financial one, since the strain of servicing it weighs measurably on wellbeing. The two biases do not just cost money; they quietly manufacture stress.
The useful response follows directly from the two mechanisms, and it is general rather than a set of instructions for any particular person’s finances. To counter exponential growth bias, make the true cost visible: look at the total interest a debt will cost over its life, not the reassuring monthly payment, because the monthly-payment framing is exactly what hides the compounding. To counter present bias, remove the temptation structurally rather than relying on willpower in the moment, since willpower is precisely what present bias defeats. None of this makes borrowing foolish in itself. It simply clarifies what the folly actually is: not debt as such, but a specific, expensive kind of it that two deep features of the mind make both easy to enter and hard to see clearly, right up until the point where it is very hard to leave.
