Who's the Most Dependable Frauds Buster?

Suzy Jagger's story in The Times newspaper online story summarizes the results of an interesting study undertaken by the university of Chicago's Graduate School of Business. I have not managed to read it but it reveals that most of the corporate fraud in a large number of corporations in the US was reported upon by lower level employees. The story states that post room workers and secretaries are more reliable in reporting fraud than are their bosses, the Securities and Exchange Commission and auditors. These lower level employees then become the natural allies of shareholders.

I wonder why those at the bottom of the corporate rung are the ones that are most prone to honesty than the rest. I surmise that the rest are also most likely to be the beneficiaries or co-conspirators in cases of corporate fraud.