Bank regulators haven’t exactly covered themselves in glory. But an earlier defense against bank excesses ought to be their boards. It’s not an easy job, but directors could try harder to control risk-taking. Uncomfortable questions are welcome.

One worth repeating, with the financial crisis slowly fading and 2009 results heralding a possible return to normality for some U.S. and European banks: “Why are we doing so well?”

Boards often discuss problems. But from the UK’s Northern Rock to Countrywide in the United States, greater scrutiny of fast-growing and highly profitable businesses might have revealed weakening standards and optimistic assumptions.

Or how about this: “What could make this institution fail?” Bank bosses should know broadly how the economy, interest rates and the like affect their business. But when the U.S. government ran stress tests last year, it found that an improbably high proportion of banks believed themselves more resilient than average.

One way to pinpoint a bank’s vulnerability — whether to a single financial asset class, counterparty or type of funding — would be to turn the stress test on its head and identify any icebergs that could sink it. Done right, that approach might have highlighted American International Group’s credit insurance portfolio — and perhaps its related and circular dependence on its credit rating.

Part-time board members can’t be expected to grasp all the ins and outs themselves. But they need to satisfy themselves the person in charge knows how the bank makes money and where its vulnerabilities are. Successive CEOs at AIG seem to have been oblivious to its Achilles’ heel. Directors also need to ask similar questions of risk managers and other employees when CEOs aren’t present.

Of course, all this is famously difficult. CEOs — who also often chair their boards, especially in the United States — can outmaneuver even highly qualified and dedicated shareholder representatives. And outside directors who ask too many tough questions soon aren’t invited back. Still, for the sake of shareholders, directors need to be as critical as possible. Maybe they need a nudge from regulators. More vigilant boards would save the watchdogs trouble.