Under a fractional reserve banking system, banks keep only a fraction of customer deposits as cash reserves and lend or invest the rest. This structure works well under normal conditions because not everyone withdraws their money at once. However, it also creates the possibility of a bank run.
How bank runs occur
Suppose a bank receives $1,000,000 in deposits.
- It keeps $100,000 as reserves (10%).
- It lends out $900,000 in mortgages, business loans, and other assets.
Even though depositors collectively believe they can access their money on demand, the bank only has a small portion available immediately. Most of its assets are tied up in loans that may take months or years to repay.
If rumors spread that the bank might fail, depositors have an incentive to withdraw their money immediately.
This creates a classic coordination problem:
- If only a few people withdraw, the bank has enough cash.
- If everyone tries to withdraw, the bank quickly runs out of liquid reserves—even if its loans are ultimately worth more than its liabilities.
A bank can therefore become illiquid (unable to meet immediate withdrawals) even while remaining solvent (its assets exceed its liabilities). If it cannot obtain additional cash quickly, it may fail.
Why bank runs spread
Bank runs often become contagious.
If one bank fails, depositors at other banks may wonder whether their institutions are also vulnerable. Even healthy banks can experience heavy withdrawals simply because people lose confidence in the banking system.
This creates systemic risk: trouble at one institution can threaten many others through panic rather than poor fundamentals.
How deposit insurance changed the incentives
The introduction of government-backed deposit insurance fundamentally changed depositor behavior.
With deposit insurance:
- Small depositors know their money is protected up to a specified limit.
- There is much less reason to rush to withdraw funds before everyone else.
- The coordination problem largely disappears for insured deposits.
For example, if deposits up to $250,000 are guaranteed, an individual with a $50,000 account has little financial incentive to join a panic. Even if the bank fails, the insured funds are expected to be repaid.
Because most retail depositors stay calm, self-fulfilling bank runs become much less common.
The role of the central bank
Deposit insurance is typically paired with the central bank acting as a lender of last resort.
When a fundamentally healthy bank faces temporary withdrawals, the central bank can lend it cash against good collateral. This allows the bank to meet withdrawals without selling long-term assets at distressed prices.
Together, these mechanisms address both sides of the problem:
- Deposit insurance reduces panic among depositors.
- Emergency central bank lending provides liquidity to banks experiencing temporary stress.
How systemic risk changed
Deposit insurance greatly reduced the likelihood of widespread retail bank runs, but it did not eliminate systemic risk. Instead, the nature of the risk changed.
Before deposit insurance:
- Systemic crises were often driven by depositor panic and cascading bank runs.
After deposit insurance:
- Retail bank runs became much rarer.
- Risks shifted toward areas such as large uninsured depositors, wholesale funding markets, highly leveraged financial institutions, and interconnected financial markets.
For example, during the 2007–2009 global financial crisis, many problems arose not from insured household depositors but from institutions that relied heavily on short-term market funding, where investors could rapidly withdraw financing.
Trade-offs
Deposit insurance has substantial benefits, but it also introduces moral hazard.
If depositors know they are protected, they may pay less attention to how risky their bank's activities are. Likewise, banks may have greater incentives to take risk if they believe depositors will not discipline them by withdrawing funds.
To offset this, governments and regulators generally combine deposit insurance with:
- Capital requirements
- Liquidity requirements
- Bank supervision and examinations
- Stress testing
- Resolution regimes for failing banks
These measures are intended to preserve the stability benefits of deposit insurance while limiting excessive risk-taking.
In summary, fractional reserve banking is inherently vulnerable to bank runs because banks promise immediate access to deposits while holding mostly illiquid assets. Deposit insurance and central bank liquidity support transformed this dynamic by reducing the incentive for depositors to panic, making traditional retail bank runs much less common and significantly lowering the risk of panic-driven, system-wide banking crises, even though other forms of systemic financial risk remain.
