So sensible policies that are needed to ensure an orderly, stable society (for example, limiting war bond repayments to a sustainable level that won't bankrupt the economy to make wealthy bondholders even richer) become impossible, and then you get societal collapse (see, for example, World War II).
The backbone of *C21* is a time-series of 300 years' worth of global capital flows, painstakingly assembled by Piketty and his grad students.
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