Financial crises come in six recurring varieties—banking crises, currency/exchange-rate crashes, sovereign defaults on external debt, sovereign defaults on domestic debt, inflation crises, and stock market crashes—and despite differences they share common quantifiable characteristics that can be identified across centuries and countries.

Because U.S. debt is denominated in its own currency (and the dollar is the reserve currency), the U.S. will likely inflate away its debts—a de facto partial default through inflation—rather than an outright de jure default, since it can simply print money to pay.

The 'this time is different' syndrome is a recurring psychological frame in which policymakers and market participants believe old valuation rules (price-earnings ratios, debt-to-income) no longer apply to them—crises happen to 'other people at other times'—which allows them to overlook mounting warning signs like real estate bubbles, unsustainable current account deficits, and off-the-chart household debt.

A common antecedent of crises across exchange-rate regimes, political systems, and budgetary situations is a 'capital flow bonanza'—running large sustained current account deficits financed by borrowing from the rest of the world, which over time makes a country increasingly indebted to the rest of the world.

Although capital inflows may arrive as equity or FDI rather than debt, money is fungible, and there is a strong empirical association across both emerging markets and advanced economies between large current account deficits/capital inflows and domestic credit availability—and the type of inflow matters for how abruptly the flow can reverse (short-term bank lending reverses brutally).

A capital flow bonanza must be defined country-specifically—it is a current account deficit that is particularly large relative to a country's own historical standard, not merely the existence of a deficit; thus England as reserve currency could run large deficits for over a century without it counting as a bonanza by British standards.

The underlying common cause of these crises is an enormous increase in credit availability that flows into asset prices—particularly housing—through the banking sector, which is a deeper driver than any single story (monetary policy, housing policy); Ireland and Spain in the eurozone (no independent monetary policy) and the UK and U.S. (floating rates) all experienced this, undercutting purely monetary-policy explanations.

As a capital inflow continues, lending quality deteriorates: in the early stages funds go into productive investment, but as inflows persist they move down the quality spectrum into more 'periphery' projects—a pattern seen in the run-up to the Asian crisis.

Crises typically follow a sequence that begins with financial liberalization and/or financial innovation, during which innovation gets ahead of regulation and supervision, creating a 'wild west' in which the boom in credit, asset prices, economic activity, and current account deficit takes seed.

The most common feature across crises is not the specific instrument or regulatory gap (which is idiosyncratic—derivatives in Mexico, securitization of mortgages in the U.S.) but that regulation and supervision consistently lag behind financial innovation; innovations like derivatives or securitization are introduced as risk-reducing but regulators are caught unaware.

Emerging markets suffer from 'debt intolerance'—they get into trouble at what appear to be low debt levels by international standards; half of post-WWII sovereign defaults occurred at debt-to-GDP ratios that would have met the Maastricht 60% criterion, so tolerable debt thresholds can be extremely low, especially without a track record.

The U.S. subprime mortgage market functioned like a developing country embedded inside the U.S.—with new entrants into credit markets who had no credit history and often no employment history—so the high default risk and confidence-fragility characteristic of emerging-market debt applied to these domestic households.

The biggest danger going forward is not what we have learned but what we will forget—the memory of the crisis will live longer for households that lost homes and jobs, but on the whole the next generation (e.g., MBAs minted in 2020 going to Wall Street) will forget, and the crisis pattern will all happen again.

The apparent higher incidence of banking crises in the recent era (vs the 1874-1913 golden age of capital mobility) is significantly an artifact of more countries in the sample—former colonies that are now independent and countries that lacked banking sectors then but have developed ones now—though the share of crises is still somewhat higher after accounting for this.

Implicit government guarantees (too-big-to-fail) are a critical amplifier of crises, linked to the debt story: if you get a great return in good states of nature and are bailed out in bad states, your willingness to borrow and take risk rises—corroborated by the fact that private debt before a crisis often becomes public debt afterward.

Central government debt rises by about 86 percent in real terms in the three years following a financial crisis, and this is a conservative estimate because it omits new guarantees that have not yet been realized (e.g., the roughly trillion dollars of Fannie and Freddie balance sheet now held by the Fed, which has not been recognized as debt).

The post-WWII period (roughly through the Bretton Woods era) was an unusually quiet period for banking crises—visible when plotting crisis incidence from 1800 to 2008—partly because international and within-system capital mobility was restrained and partly because much of the world was rebuilding real, productive 'low-hanging fruit' after wartime destruction.

Asia ran huge current account deficits before the 1997-98 crisis, much of it financed by foreign direct investment rather than pure debt flows, yet still suffered crisis—showing that even non-debt inflows correlate with the credit availability that precedes crises.

Credit availability is the necessary seed and moral hazard from implicit guarantees is the amplifier: without ample credit availability, moral hazard is largely moot because you can't borrow regardless of guarantees; with it, guarantees create 'hidden debt' by raising the demand for credit from economic actors who expect a government bailout, and displace lending quality toward the lower end.

Countries can run current account deficits or surpluses for extended periods, and these flows move in long cycles—England ran current account deficits and capital account surpluses for about 120 years as the world's dominant lender, which would look like a huge deficit by another country's standards but did not qualify as a capital flow bonanza by British standards.