Capital — machinery, financial assets, real estate, intellectual property — generates a return. That return, reinvested, generates more capital. Compound this over decades and the initial advantage becomes enormous. A family that enters the 20th century with substantial capital likely exits it with far more, relative to families that entered with wages only. The mathematics is mechanical; the politics is not.

The r > g Formula

Thomas Piketty's central argument in Capital in the Twenty-First Century (2014) is that the return on capital (r) historically exceeds economic growth (g). When r > g, capital income grows faster than wage income, and wealth concentrates. This is not controversial as a mathematical fact. The contested question is whether it describes an iron law or a historical tendency that policy can modify.

Piketty's empirical work, drawing on tax records reaching back to the nineteenth century, shows that r > g held for most of recorded history with one major exception: the period from roughly 1914 to 1980, when two world wars destroyed European capital stocks and progressive taxation in much of the developed world compressed top-end wealth. The re-emergence of r > g dynamics since 1980, his data suggests, is a return to the long-run trend rather than a deviation from it.

What Slows Concentration

Three forces have historically counteracted concentration. Wars destroy capital — both physically and through the redistributive tax regimes that followed major conflicts. Progressive taxation, estate taxes, and capital gains taxes reduce the effective return on accumulated wealth. And broad access to education increases the supply of human capital, compressing the wage premium for skills.

All three of these have weakened since 1980. Top marginal tax rates have fallen across most developed countries. Estate taxation has been hollowed out by exemptions and loopholes in the US and abolished outright in several European countries. Educational attainment has continued to rise but the wage premium for advanced credentials has risen faster, not slower — suggesting that the relevant scarcity has shifted to narrower categories of human capital that broad expansion of schooling does not reach.

The Market-Failure vs. Policy-Failure Readings

Progressives read rising inequality as evidence that capitalism's returns structure is fundamentally broken — that capital markets systematically overreward owners. Conservatives read the same data as evidence of policy failure: tax regimes that favor capital income over labor income, zoning that captures land value for existing owners, and regulatory capture that protects incumbent capital. Both readings contain truth. The structural dynamics of compounding and the specific policy choices that amplify or dampen them are both real and separable.

The empirical literature on this — Saez and Zucman's tax-data work, the World Inequality Database project, the OECD's wealth statistics — has generally found that policy explains a large share of the variance across countries with comparable demographic and technological conditions. The US and France have similar productivity and similar demographic structures; their wealth-concentration trajectories diverged sharply over four decades because their tax and transfer regimes diverged.

The Wage Stagnation Connection

The flip side of capital concentration is that wages, especially median wages, decoupled from productivity growth somewhere around 1979 in the US. Productivity has roughly doubled since then; the median male wage has risen only modestly. The Brookings Institution and EPI have documented this in extensive detail, and the underlying causes — declining union density, the China shock, the rise of monopsony in local labor markets, the shift of bargaining power to capital — are individually contested but collectively pointing in the same direction. When wages stagnate while capital returns compound, the wealth distribution must become more concentrated. The arithmetic is unavoidable.

Inherited Wealth and Social Mobility

The Great Gatsby Curve — a cross-country correlation between income inequality and the persistence of family income across generations — was named and documented by Alan Krueger and refined by Miles Corak. The finding is that countries with higher inequality also have less intergenerational mobility, which means that the contemporary wealth gap predicts the inherited wealth gap of the next generation with high fidelity. This is the central empirical claim behind the Piketty worldview: not just that wealth is unequal now, but that the inequality is increasingly inherited rather than earned.

The Honest Reading

Capital accumulation as a mechanism is morally neutral — it is arithmetic. The political question is whether the resulting wealth distribution is compatible with the political institutions a society wants to maintain. Democratic legitimacy depends in some loose way on the perception that the economic system is fair, and runaway concentration corrodes that perception. The countries that have maintained both relatively high capital accumulation and relatively broad wealth distribution — Sweden, Denmark, Netherlands — have done so through specific policy choices that did not happen by accident. The choice to make those choices, or not to, is what the political economy of capital accumulation is actually about.

Why It Compounds Faster Now

Several features of the modern economy accelerate the compounding dynamic. Asset valuations have risen faster than incomes for four decades. Tax treatment of capital income remains more favorable than the treatment of wage income. The interaction of inheritance and asset appreciation produces wealth dynasties that the early-twentieth-century estate-tax framework was designed to interrupt and that the late-twentieth-century rollback of that framework has allowed to re-emerge. The mechanism is mechanical, but the policy choices that determine its speed are political. Whether the next decade produces serious rethinking of capital taxation or whether the current arrangements compound for another generation is one of the central distributional questions of the 2020s.

The Generational Compounding

The wealth distribution at any point in time predicts the wealth distribution one generation later with high fidelity. Children of wealthy parents inherit advantages in education, networks, financial cushioning, and outright transfers that compound their own subsequent accumulation. The cross-country evidence (Miles Corak, Raj Chetty) shows that this intergenerational persistence is strongest in the highest-inequality countries — the US, the UK, Italy — and weakest in the lowest-inequality countries — Denmark, Norway, Finland. The mathematical compounding of wealth across generations is a structural feature of capitalism; whether it produces a hereditary upper class or a broadly meritocratic society depends on the institutional choices that either accelerate or interrupt the compounding.