The provision of non-audit services (NAS) to audit clients can generate knowledge spillovers that enhance auditors' judgments or self-review and self-interest threats that impair auditors' independence. Prior research finds mixed evidence of a relation between tax NAS and clients' (actual and potential) material GAAP violations in accounting for income taxes. As auditors are likely to avoid material GAAP violations, we re-examine this issue using a measure that reflects immaterial or within-GAAP estimation error in clients' income tax expense. We find that greater amounts of tax NAS are associated with greater income tax estimation error, consistent with tax NAS threating auditors' independence. The association is partially offset by auditor expertise and concentrated in engagements where auditors face both self-review and self-interest threats. Our findings inform the ongoing policy debate regarding whether accounting firms should provide tax NAS to their audit clients.

Data Availability: Data are publicly available from the sources indicated in the text.

JEL Classifications: M41; M44; M49.

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