The life-cycle theory of savings and personal finance

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How you save and spend depends a lot on your age.

© TKDmitry Vavilon, mika, Youttakone, Luke/stock.adobe.com; Photo illustration Encyclopædia Britannica, Inc.

In the 1940s, economist Franco Modigliani was researching how increases in income affect economic growth, and he was struck by how variable it was. It wasn’t clear how much a change in income would translate to changes in consumer spending and savings, and that made economic forecasting difficult.

As he thought about it, Modigliani realized that consumers save and spend differently—but in predictable ways—in different phases of their lives. In 1954, he published a paper with one of his graduate students, Richard Brumberg, that set out a theory of how spending varies over a lifetime. Modigliani eventually called it the “Life Cycle Hypothesis of Savings,” more commonly known as life-cycle theory.

Life-cycle theory was only one of the significant contributions that Modigliani made to economics. (He won the Nobel Prize in 1985 for his work in the field.) Life-cycle theory is logical, doesn’t require a background in economics to understand, and applies to personal finance and global economic development alike.

Modigliani and Brumberg started with a concept of life resources: that is, the present value of all income and gifts received over a lifetime. These resources are not received evenly, but they are more or less spent evenly. Throughout our lives, we need to take care of the basics like groceries and utility bills, and we probably want to have some extras. But our incomes vary wildly over time. Debt and investments allow consumers to manage their consumption as their income varies.

Most consumers make no or little money until they enter young adulthood. The income from their first jobs rarely covers all their expenses. Most people’s income and wealth peak in late middle age, when the average person has a higher income, pays down the debt accumulated during youth, and saves aggressively for retirement. Then, in retirement, they spend down their savings.

Modigliani and Brumberg noted that consumers move through different phases as they age, with different levels of income and spending:

The life-cycle theory was developed for economic forecasting, not financial planning, but it gives consumers a way to think about how their income and expenses will change over time. If you know you’ll probably have debt early in your career, you can focus on finding the best interest rates. As you start to make more money, you can shift to paying off that debt instead of continuing to borrow. Although it’s better to start saving for retirement early, there’s still time in middle age to accumulate resources. In retirement, the focus shifts to spending responsibly to preserve the nest egg.

Of course, your individual experience may be very different from the average. Some children have inherited income, while others work in family businesses. Some workers make the bulk of their income early in their careers, while others never really retire and so never really enter the spending phase.

Modigliani and Brumberg were looking to improve economic forecasting, and the life-cycle theory works well for that. Because people at different stages of their lives respond differently to an increase in income, changes in the economy affect them differently. Those in the consumption phase have no income, so an economy-wide increase in income won’t affect them directly.

Consumers in the accumulation phase do spend money, so money that comes their way is likely to be spent on necessities, including durable goods such as cars and appliances. A consolidator is likely to use increased income to pay down debt and increase investments, while someone in the spending phase is likely to use money to pay for necessities.

Likewise, a nation early in its industrial development might have considerable business and consumer debt because it’s building infrastructure. A nation with an older population may be disinvesting because its population is spending money accumulated at an earlier age.

Modigliani himself noted many of the criticisms of the life-cycle theory. A significant one is that it doesn’t account for family size; a family with several children may spend a far greater proportion of its income in the parents’ middle age than in any other phase of life. Another is that it doesn’t accommodate an increase in life spans, which alters the length of each stage and may affect how consumers behave. Finally, it doesn’t accommodate inheritances well, either from the perspective of someone receiving one or the person who is hoping to leave one.

The life-cycle theory is an important concept for economic analysis, and it offers some insight into personal financial planning over a lifetime. After all, you have one life, although the circumstances within it change. One common budget guideline, the 50/30/20 rule, recommends that your budget be allocated 50% toward needs, 30% toward wants, and 20% toward savings (which includes paying down debt). The life-cycle theory may help you think about how to adjust these allocations in the future while also managing your life right now.