A compelling case can be made for mandatory audit rotation that auditors who keep the same client for too long get excessively cosy with its management. As somebody has put it, “When the same incumbent firm has been in place for 100 years, to me that’s not an audit, that’s a joint venture.”

Most academic studies have either found no link between the length of a relationship and its quality, or determined that longer tenures yield better results, because the accountants have time to master the intricacies of clients’ businesses. Obliging companies to solicit bids at regular intervals, as Britain does, has shaken up the business: HSBC said in August that it will drop KPMG in favour of PwC; on December 2nd Unilever announced that it is making the opposite switch. Indeed, the big accounting firms argue that forced rotation would reduce competition by preventing the incumbent from bidding.

Proponents counter that the accounting giants, and academics whose research is often financed by them, have good reason to resist change. Among the reform’s strongest supporters are smaller firms that hope to break the Big Four’s stranglehold.

Yet even the most vocal advocates of mandatory rotation concede that it is no magic bullet. Auditors have a conflict of interest at the heart of their business — they are paid by the companies they are supposed to assess objectively. Unless that changes, there will be no substitute for investors doing their own due diligence.