We commonly assume that commercial banks are regulated and stick to the safe stuff while the investment banks exist for the riskier bond and currency trading. Not so, reports Forbes, because accounting rules give commercial banks a perverse incentive to hold on and keep the bet going where investment banks have to take the hit and move on.

Investment banks have to mark all positions to market every day and so they can’t fudge the numbers. Commercial banks have three choices to classify their riskier investments and two of the three do not affect the bottom line. They can hang on and on hoping a losing position will turn around.

“The ability to place trades in different buckets allows you to ignore the problem,” Brad Hintz, an analyst at Sanford Bernstein, tells the magazine. “And the ability to ignore problems can lead to much worse problems. Mark-to-market accounting forces you to take the hit” and get out while you can.

Other analysts agree that the rules make give banks incentives to do riskier things. The only saving grace is that commercial banks usually get 10 percent of their income from such trading where the investment banks earn half their profits from the riskier activities.