Wall Street would like you to believe that when investment banks take on risk they are largely doing it for the benefit of investors -- maybe even you and me.

Bankers say much of the capital that their firms put at risk each day is to complete trades for big corporations, mutual funds, pension funds, hedge funds and university endowments. And contrary to the conventional wisdom, proprietary trading -- bets made for a bank's own behalf -- is really just a small part of their business.

Lately, Wall Street's captains of capitalism have been aggressive in pushing the "we take big risks for our customers, not for ourselves" line of argument.

That's especially so in the wake of the public furor over the outsized trading gains at the big banks like Goldman Sachs Group, JPMorgan Chase and Barclays and even Citigroup, so soon after the collapse of Lehman Brothers.

The notion that risk is being taken for customers as opposed to for the firm's own benefit is somehow supposed to make it seem more palatable and somehow less risky.