This paper presents the first causal evidence on how banks adjust their voluntary capital buffers (the capital headroom above the required level) in response to changes in capital requirements. Using granular euro area data and exploiting the threshold-based assignment of Other Systemically Important Institution (O-SII) buffers within a regression discontinuity design, we study the liability side of banks’ balance sheets, complementing the asset-focused literature on lending and risk-taking. This allows us to assess whether capital regulation is effective in enhancing bank resilience, arguably its main objective. We find that banks offset about half of higher capital requirements by cutting their voluntary buffers rather than raising new equity. The offsetting effect is more pronounced among banks with weaker balance sheets, particularly those with higher levels of non-performing loans. These results indicate that regulation aimed at strengthening resilience may be only partially effective, as banks use existing voluntary buffers when subject to higher requirements.
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