Beta risk (practitioner concept)
Definition
Not a formal regulatory risk type. The risk that the assumed rate of pass-through from market rates to managed product rates (the deposit beta) is wrong, unstable, or non-linear — and that this mis-estimation materially affects NII, EVE, or both.
What This Actually Means
Deposit beta (pass-through rate) is the assumption that determines how much of a rate move gets passed on to depositors on managed rate products. A beta of 0.5 means if rates rise 100bps, deposit costs rise 50bps — the bank retains the other 50bps as margin.
Beta risk is the risk that this assumption is wrong. The beta may be higher than modelled (competitive pressure forces more pass-through than expected, compressing margin), lower than modelled (deposit inertia means the bank retains more margin than assumed), non-linear (the beta behaves differently at different rate levels — compressing toward zero at the lower bound, potentially accelerating in a sharp rate spike), or unstable over time (the competitive landscape shifts and historical betas stop being a reliable guide).
This is not a risk type recognised in the EBA IRRBB framework, which distributes beta-related assumptions across gap risk (NMD repricing) and option risk (behavioural floors). But in practice, for most retail banks, the beta assumption is the single largest driver of NII uncertainty — larger than gap risk in many rate environments.
Where It Matters
Beta risk sits at the intersection of gap risk, basis risk, and option risk — which is precisely why it doesn't fit neatly into the regulatory taxonomy and tends to be under-scrutinised as a result. Beta uncertainty is not really a timing mismatch (gap risk), an index mismatch (basis risk), or a contractual optionality problem (option risk). It is the risk that a discretionary management and customer behaviour assumption — how much rate to pass through — turns out to be wrong. That is a distinct risk with its own drivers and its own governance requirements.
Where it bites hardest: in the 2022-23 rate hiking cycle, banks that had modelled low deposit betas based on the post-2008 low-rate experience found those betas rising sharply as competitive dynamics changed and customers became more rate-aware. NII projections that looked robust under standard scenarios proved optimistic. The beta assumption had been set once, validated against a benign historical period, and not stress-tested adequately for an environment where depositors actually cared about rates.
Impact on structural hedge size: the beta assumption also directly determines the notional size of the structural hedge. The stable, non-rate-sensitive portion of the deposit base — the volume that is deemed unlikely to reprice or migrate — is estimated in part using the beta. A lower assumed beta implies more stable, longer-duration deposits, supporting a larger hedge notional. If the beta turns out to be higher than modelled, the deposit base is less stable than assumed, and the structural hedge is oversized relative to the actual exposure — creating a hedge mismatch that can itself become a source of P&L volatility. This is a second-order consequence of beta mis-estimation that is often not surfaced in standard sensitivity analysis.
Governance implication: because beta assumptions span multiple risk categories, ownership is often fragmented — the ALM model owns the EVE repricing profile, the NII model owns the pass-through assumption, and neither is explicitly stress-tested as a standalone risk. Naming it explicitly, even as a practitioner concept, helps ensure it gets owned and challenged.