The Influence of Assurer Type, Standard Assurance, and Level Assurance on the Extent of Assurance Statements in Sustainability Reports: A Conceptual Study

This study conceptually examines the influence of assurer type, assurance standards, and assurance level on the breadth of assurance statements in sustainability reports. Moving beyond prior literature that treats assurance as a binary variable (presence versus absence), this paper highlights disclosure breadth as a critical dimension of assurance quality and substance. Drawing on legitimacy theory and complemented by institutional theory, the study argues that the technical configuration of assurance shapes the quality of organizational legitimacy obtained by firms. Specifically, the type of assurer (public accounting firms versus non-accounting providers), the standards adopted (e.g., ISAE 3000 and/or AA1000AS), and the level of assurance (limited versus reasonable) influence the structure, systematic presentation, and comprehensiveness of assurance statements. Firms that engage reputable providers, apply globally institutionalized standards, and select reasonable assurance are more likely to issue broader and more detailed statements. In contrast, weaker institutional pressures may encourage symbolic assurance practices characterized by minimal disclosure. The study contributes theoretically by extending legitimacy theory to the technical dimensions of assurance and positioning disclosure breadth as a proxy for substantive legitimacy. Practically, it suggests that regulators and companies should emphasize transparency and comprehensiveness in assurance statements to enhance credibility and discourage symbolic sustainability reporting practices.