Concentration Risk: Measurement and Limits

1. Why Concentration Risk Matters

Concentration risk arises when credit portfolios lack diversification across borrowers, sectors, or geographies. A single adverse event—borrower default, industry downturn, regional economic shock—triggers correlated losses exceeding expectations from standard credit risk models that assume independence.

Regulatory frameworks (Basel III Pillar 2, Resolution 4.557 in Brazil) mandate explicit concentration risk assessment within ICAAP. Institutions must quantify concentration beyond Pillar 1 capital requirements and hold additional capital or impose portfolio limits to mitigate tail risk.

2. Types of Concentration Risk

• Single-name concentration: Large exposures to individual borrowers or connected groups. Regulatory large exposure limits (typically 25% of capital) provide hard caps, but prudent management sets lower internal thresholds.
• Sectoral concentration: Overexposure to industries (real estate, energy, agriculture) vulnerable to common shocks. Economic cycles amplify correlation within sectors.
• Geographic concentration: Concentration in specific regions or countries exposed to localized disasters, regulatory changes, or economic downturns.
• Product concentration: Overreliance on single product type (e.g., 80% residential mortgages) creates vulnerability to product-specific risks (housing bubble, regulatory shifts).
• Collateral concentration: Secured portfolio backed predominantly by one asset class (real estate) amplifies risk during collateral value declines.

3. Herfindahl-Hirschman Index (HHI)

HHI quantifies portfolio concentration via sum of squared market shares:

Formula: HHI = Σ(share_i)² where share_i is exposure to entity/sector/region i as fraction of total portfolio.

• Interpretation: Ranges from 1/N (perfect diversification across N equal exposures) to 1 (complete concentration in single exposure).
• Benchmark thresholds:
  • HHI < 0.15: Low concentration (competitive market standard).
  • HHI 0.15-0.25: Moderate concentration—warrants monitoring.
  • HHI > 0.25: High concentration—supervisory concern, likely Pillar 2 capital add-on.
• Practical calculation: Compute HHI for borrower, sector, and geography dimensions separately. Report trend over quarters to detect concentration drift.

4. Granularity Adjustment for Capital

Basel IRB framework includes granularity adjustment reducing capital for well-diversified portfolios or increasing it for concentrated exposures:

• Calculate effective number of exposures: N_eff = 1 / HHI.
• Adjustment formula penalizes portfolios with N_eff < 100 (concentrated) via multiplicative factor applied to IRB capital.
• Institutions with N_eff > 1000 (highly granular retail portfolios) benefit from reduced capital charges.

Supervisors may impose additional Pillar 2 charges if internal models underestimate concentration risk or if HHI trends upward.

5. Concentration Limit Frameworks

Single-name limits:

• Regulatory ceiling: 25% of Tier 1 capital per borrower/group (Basel large exposure standard).
• Internal appetite: Conservative institutions set 10-15% limits; differentiate by counterparty rating (higher limits for investment-grade).
• Approval hierarchy: Exposures >5% capital require board approval; >10% need quarterly monitoring.

Sectoral limits:

• Define maximum exposure per sector as % of total portfolio (e.g., real estate <30%, energy <15%, agriculture <10%).
• Calibrate limits based on historical sector-level default correlations and stress test losses.
• Review annually; tighten during sectoral stress (commodity price collapse, housing downturn).

Geographic limits:

• Cap exposure to individual states/regions based on GDP share and economic diversification.
• Emerging market exposures often subject to country risk limits (foreign exchange, sovereign risk, transfer risk).

6. Stress Testing Concentration Risk

Standard credit risk models underestimate tail losses from concentrations. Dedicated stress tests reveal capital adequacy gaps:

• Single-name default scenarios: Assume top-3 borrowers default simultaneously with LGD 50-75%. Calculate capital depletion and CET1 ratio impact.
• Sectoral shock scenarios: Model oil price collapse affecting 20% of portfolio (energy sector), doubling default rates and halving collateral values.
• Geographic disasters: Hurricane/flood affecting regional portfolio with 30% delinquency spike and property value declines.
• Reverse stress testing: Identify concentration levels that would deplete capital buffers—informs preemptive limit tightening.

7. Monitoring and Reporting

Monthly dashboards:

• Top-10 borrower exposures with utilization vs. internal limits.
• Sectoral HHI trends and exposures exceeding risk appetite thresholds.
• Geographic breakdown with regional economic indicators (GDP growth, unemployment).
• Early warning: Exposures approaching limits trigger automated alerts to credit officers.

Quarterly governance:

• Credit committee reviews concentration metrics; approves limit exceptions or remediation plans.
• Pillar 2 capital allocation: Quantify concentration risk capital charge using internal models or supervisory multipliers.
• Board reporting: Summary of concentration trends, limit breaches, corrective actions.

8. Mitigation Strategies

• Portfolio diversification: Proactive origination in underweight sectors/regions; participation in syndications to avoid large single-name exposures.
• Credit derivatives: Purchase single-name CDS or sector index CDS to hedge concentration without asset sales (subject to counterparty risk).
• Loan sales and securitization: Reduce concentrated exposures via secondary market transactions or securitization (retain senior tranches to maintain client relationships).
• Collateral requirements: Demand additional collateral or guarantees from concentrated borrowers to reduce LGD.
• Dynamic limits: Tighten limits preemptively when sector outlook deteriorates (e.g., energy limits cut as oil prices decline).