The forensic factor is starting to lose its power to shock. Just look at Fannie Mae, which has unveiled yet another restatement.

By the standards of the US mortgage giant, it is benign stuff: a mere $6.7bn in loans purchased last year, which it accidentally classified as multi-, rather than single-family mortgages. That should be earnings neutral and had little impact on its share price. At about a third of Fannie Mae’s originally reported purchases of multi-family mortgages, however it is not the sort of error you would want to make on your loan application.

Fannie Mae is not the only one to say sorry. Over the past few weeks alone, Krispy Kreme, Ameritrade, American International Group and General Motors have all restated financial data. In part, this reflects boards and auditors feeling understandably worried after recent scandals. Once forensic accountants have been called in, moreover, they are likely to eventually discover something. Businesses and accounting rules alike have steadily grown more complex, increasing the scope for error. Mistakes may often be genuine – after all, historic earnings are occasionally revised upwards.

Investors have been quite forgiving lately, particularly with serial restaters. This applies especially to companies already in trouble, such as GM and Krispy Kreme, and to those, such as Fannie Mae and AIG, whose finances are hard to understand to begin with. However, the lack of market reaction to many restatements is a concern. A they become more numerous, they lose their effectiveness as a signal for investors.

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