Demographics Are Destiny, With a Forty-Year Lag

The number of people who will turn forty in 2065 is already fixed by the number of people born in 2025. This is close to the only variable in macroeconomics that can be forecast with near certainty, and it is routinely absent from investment frameworks because its horizon exceeds every professional evaluation period that exists.

The mechanism through which demographics affect asset prices runs through the dependency ratio, the number of non-working people supported by each worker. A falling dependency ratio, which occurs when a large cohort enters working age, generates a demographic dividend: high savings rates, abundant labour, and strong growth in aggregate output. A rising dependency ratio reverses each of those conditions.

The savings-rate inflection

Individuals accumulate savings during working life and draw them down in retirement. When a large cohort is in its peak earning years, the aggregate savings pool grows, which suppresses the real interest rate and inflates the price of long-duration assets. When that cohort retires, the flow reverses: the pool is drawn down rather than added to, and the same mechanism that suppressed rates for decades operates in the opposite direction.

The developed world crossed this inflection at different times in different countries, with Japan first by a wide margin. Anyone seeking evidence for how the transition proceeds has three decades of Japanese data to examine, which is more information than most macro theses ever get.

Why migration changes the arithmetic but not the direction

Migration alters a national dependency ratio without altering the global one, which means it redistributes the demographic dividend rather than creating it. For an individual economy this distinction is academic and the effect is real: Australia has sustained working-age population growth through migration at rates that have materially offset domestic fertility decline, which is a genuine difference from the Japanese or Italian trajectory.

The constraint is that migration-led population growth requires the physical and social infrastructure to absorb it, and the political sustainability of the policy depends on that absorption succeeding. This makes the Australian demographic outlook less a matter of arithmetic and more a matter of infrastructure delivery and political consent, both of which are considerably less predictable than a birth cohort.

Why the Prediction Is Reliable and the Trade Is Not

Knowing the direction of a variable over forty years does not tell you what any asset will do over the next four. The demographic thesis for lower real rates was correct for three decades and then failed comprehensively in 2022, when an inflation shock overwhelmed the slow variable entirely. Demographics did not stop operating. They were simply not the dominant force in that window.

This is the general problem with long-cycle frameworks: they describe the tide and not the waves, and portfolios are marked to market against the waves. The reasonable use is to let demographics inform the direction of strategic tilts held for decades, while accepting that any horizon short enough to matter for performance reporting will be dominated by faster variables.

The Japanese laboratory

Japan crossed the working-age population peak in 1995 and offers three decades of data on what happens afterward. Real yields fell to zero and stayed there. Domestic equity multiples compressed for a generation. Household savings pools that had accumulated during the demographic dividend began drawing down rather than adding, which forced Japanese institutions to search offshore for return and turned Japan into the largest foreign holder of United States Treasuries in the world.

Each of these outcomes is directly attributable to the demographic transition. None of them was priced in advance. That gap between what was predictable in principle and what got priced in practice is the interesting feature of the case, and it recurs in every country that has since followed Japan through the same threshold.

The Australian mitigating factor and its limit

Australia has offset domestic fertility decline through sustained working-age migration at rates that would be politically impossible in most comparable economies. The result is a national dependency ratio that has stayed materially more favourable than the demographic arithmetic implied by births alone would suggest. This is a genuine difference from the Japanese trajectory and one of the underappreciated structural supports for Australian growth and asset prices over the past thirty years.

The mitigating factor is not unlimited. Migration-led population growth requires physical and social infrastructure to absorb it, and the political sustainability of the policy depends on that absorption succeeding. When infrastructure delivery lags population growth by enough, the political consent that underwrites the migration rate becomes uncertain, which is a domestic policy question rather than a demographic one, and it is the variable most worth watching for anyone whose thesis on Australia rests on the mitigating factor continuing to operate.