Banks, by contrast, routinely have liabilities in excess of 90 percent of their assets. JPMorgan Chase’s $2.2 trillion in debt represented some 91 percent of its $2.4 trillion in assets. (Under accounting conventions used in Europe, the figure would be around 94 percent.)

Basel III would permit banks to borrow up to 97 percent of their assets. The proposed regulations in the United States — which Wall Street is fighting — would still allow even the largest bank holding companies to borrow up to 95 percent (though how to measure bank assets is often a matter of debate).

If equity (the bank’s own money) is only 5 percent of assets, even a tiny loss of 2 percent of its assets could prompt, in essence, a run on the bank. Creditors may refuse to renew their loans, causing the bank to stop lending or to sell assets in a hurry. If too many banks are distressed at once, a systemic crisis results.

Prudent banks would not lend to borrowers like themselves unless the risks were borne by someone else. But insured depositors, and creditors who expect to be paid by authorities if not by the bank, agree to lend to banks at attractive terms, allowing them to enjoy the upside of risks while others — you, the taxpayer — share the downside.

Implicit guarantees of government support perversely encouraged banks to borrow, take risk and become “too big to fail.” Recent scandals — JPMorgan’s $6 billion London trading loss, an HSBC money laundering scandal that resulted in a $1.9 billion settlement, and inappropriate sales of credit-card protection insurance that resulted, on Thursday, in a $2 billion settlement by British banks — suggest that the largest banks are also too big to manage, control and regulate.

NOTHING suggests that banks couldn’t do what they do if they financed, for example, 30 percent of their assets with equity (unborrowed funds) — a level considered perfectly normal, or even low, for healthy corporations. Yet this simple idea is considered radical, even heretical, in the hermetic bubble of banking.

Bankers and regulators want us to believe that the banks’ high levels of borrowing are acceptable because banks are good at managing their risks and regulators know how to measure them. The failures of both were manifest in 2008, and yet regulators have ignored the lessons.