We offer an economic explanation for why audit firms oppose mandatory firm rotation. Using an innovative sample that overcomes sample selection biases, we find that fees for Big 4 audit firms increase noticeably over the audit firm's tenure. In contrast, fees for non-Big 4 audit firms decline as tenure lengthens. Using audit report lag as a proxy for audit cost, we find that audit cost declines over the audit firm's tenure, and this decline is even larger for Big 4 auditors. Our results indicate that Big 4 engagements become more profitable or earn “quasi rents” over time, which may explain why Big 4 audit firms are so opposed to firm- but not partner-rotation. Whether non-Big 4 auditors earn any quasi rents remains doubtful. Our findings suggest a need to better monitor auditor independence and audit judgments when tenure is long, especially for Big 4 auditors, because economic bonding between the audit firm and client tends to increase over time.

JEL Classifications: M40; M42.

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