Choosing Over Time: Saving and Borrowing

How economists model decisions about spending now versus later, the role of the interest rate, and why people smooth consumption over their lives.

Many economic choices involve time: saving for the future, borrowing today, or investing now for later returns. Economists call these intertemporal choices.

The basic trade-off

Money saved today can earn interest and be spent later. Money borrowed today must be repaid with interest later. The interest rate is the price of moving money across time.

The economist Irving Fisher developed a framework in the early twentieth century showing how people choose between consumption today and consumption tomorrow, given their income in each period and the interest rate.

Consumption smoothing

People generally prefer a steady standard of living rather than feasting in some years and going hungry in others. This desire is called consumption smoothing. People save when income is high and borrow or draw on savings when income is low.

The life-cycle hypothesis

Economist Franco Modigliani, who won the Nobel prize in 1985, developed the life-cycle hypothesis: people plan consumption over their whole lives. They may borrow when young, for education or homes, save during their working years, and spend savings in retirement.

The permanent income hypothesis

Milton Friedman argued that people base spending on their permanent income, their expected long-run average income, rather than current income alone. A temporary windfall may be mostly saved, while a permanent pay rise leads to higher spending.

Real-world limits

Borrowing constraints: many people, especially poorer households, cannot borrow easily, so they cannot smooth consumption as theory suggests.
Present bias: people may overweight the present and under-save, as behavioural economics shows.
Uncertainty about future income.

Interest rates and behaviour

Higher interest rates reward saving and make borrowing costlier, which is one way central banks influence spending.

A farmer's year

A farmer receives most of his income at harvest time but needs to buy food all year. He saves part of the harvest income and spends it gradually over the following months. If harvest is poor, he borrows or draws on savings. This is consumption smoothing in practice, and access to savings and credit makes it easier.

Thinking people spend based only on this month's income

Theory and evidence suggest people consider expected future income, though borrowing limits and present bias mean many cannot or do not smooth consumption fully.

Key takeaways

Intertemporal choices involve spending now versus later, with the interest rate as the price of time.
People prefer to smooth consumption over time.
Modigliani's life-cycle hypothesis and Friedman's permanent income hypothesis describe long-run planning.
Borrowing constraints and present bias limit consumption smoothing in reality.