How Bank Runs Start and Spread

A bank run is a self-fulfilling panic: depositors believe a bank is insolvent, so they rush to withdraw their money at once, which drains the bank’s cash and makes it insolvent whether or not the fear was justified to begin with. The mechanics are simple, but the conditions that trigger them—and the speed at which panic spreads—have shaped financial crises for centuries.

The Core Mechanism: Illiquidity Becomes Insolvency

A bank takes deposits (liabilities) and makes loans and investments (assets). As long as depositors believe their money is safe, they leave it in the bank. The bank can then lend that money out at a higher rate of interest, earning a spread. This arrangement is called fractional reserve banking—the bank keeps only a fraction of deposits in cash or near-cash instruments; the rest is deployed into illiquid assets like mortgages, corporate loans, or securities.

Fractional reserve banking works fine as long as the flow of deposits and withdrawals is orderly. On any given day, some customers withdraw cash and others deposit it, and the flows roughly balance. The bank can meet withdrawal requests from its cash reserves and new deposits. It can also borrow short-term from other banks if it needs temporary liquidity.

But the system has a fatal vulnerability: it cannot handle simultaneous withdrawal requests from all depositors. If a significant fraction of depositors try to withdraw their money at the same time, the bank’s cash reserves run out immediately. The bank then faces a choice: sell assets quickly (which may be worth less if sold in haste), borrow emergency funds (which may not be available if other banks are also under stress), or close and trigger liquidation.

Why the Panic Starts: Signals and Triggers

Bank runs are often preceded by a specific event or revelation that changes depositor confidence. The trigger might be genuine financial distress: news that a bank’s loan portfolio has deteriorated, that management committed fraud, or that the bank has made catastrophic investment bets. It might also be exogenous: a crisis elsewhere in the financial system that spooks depositors into withdrawing funds from multiple banks at once.

The key insight is that the trigger doesn’t have to be true. If depositors believe a bank is in trouble, and if they believe that other depositors will also try to withdraw, then each depositor has an incentive to withdraw immediately—before the cash runs out. This is a collective-action problem. Individually, if you think a bank is sound, you’d leave your money there. But if you think others will panic, you’d withdraw even if you privately believe the bank is solvent. Your withdrawal contributes to the run, proving the panic right.

During the Great Depression, bank runs started when rumors spread that a particular bank had made bad loans or that a prominent depositor had withdrawn large sums. In the 1980s U.S. savings-and-loan crisis, the trigger was rising interest rates that created losses on bond portfolios. In March 2023, Silicon Valley Bank’s collapse was triggered by news that it had suffered unrealized losses on a large portfolio of Treasury securities and that it needed to raise emergency capital to cover a deposit outflow.

Contagion: How One Run Triggers the Next

A bank run at one institution can spread to others. This happens through multiple channels:

Information asymmetry and guilt by association. When one bank fails, depositors at other banks ask themselves: are my deposits safe? If two banks seem similar in some way—they operate in the same industry, they have similar loan portfolios, they’re in the same geographic market—then news of one bank’s failure triggers a reassessment of the other. During the 2023 SVB collapse, depositors at other regional banks that held large portfolios of duration-sensitive securities rushed to withdraw, fearing the same losses.

Wholesale funding stress. Many banks fund themselves through wholesale markets—they borrow from other banks, money market funds, and securities dealers, not just from depositors. When one bank enters distress, credit lines from other banks may be withdrawn, and term funding may become unavailable at any price. This forces other banks to liquidate assets or call in loans to raise cash, which can trigger their own runs if they can’t meet demand.

Reserve depletion and fire sales. When a run-on-one bank forces it into rapid asset sales, those sales drive down prices for similar assets. Other banks holding the same assets see their balance sheets deteriorate. If they’re forced to mark-to-market those losses, their capital ratios fall, which triggers regulatory concerns and depositor panic at those banks too.

Deposit insurance limits and moral hazard. In most developed countries, deposit insurance protects small depositors up to a limit (in the U.S., $250,000 per account per bank at the Federal Deposit Insurance Corporation). Large depositors and uninsured deposits—particularly large corporate accounts and municipal treasuries—can be lost entirely if a bank fails. This creates a two-tier response to bank failure news: insured depositors may remain calm, but uninsured depositors often flee en masse. This dynamic was critical in the 2023 SVB run, where much of the bank’s deposit base consisted of venture capital funds and tech companies with balances far above the insurance limit.

Historical Patterns: The 1930s and 2008

The Great Depression provides the clearest case study. From 1930 to 1933, thousands of U.S. banks failed, many of them triggered by local bank runs. Runs started when rumors of bank trouble circulated, often based on accurate information about loan defaults in agricultural regions. The runs themselves then cascaded: when one bank failed, depositors at nearby banks panicked. Federal Reserve policy actually worsened the spiral—the Fed tightened credit conditions in the early 1930s, believing that failing banks deserved to fail, which made it harder for solvent banks to borrow liquidity and survive runs.

In 2008, the financial crisis saw a mirror image dynamic. Many banks were insolvent or nearly so because of mortgage losses, but the threat of old-fashioned depositor runs was prevented by massive Federal Reserve lending programs and by emergency increases in deposit insurance. Instead, the runs happened in the shadow banking system: hedge funds, money market funds, and securities dealers engaged in “runs” by not rolling over short-term funding, forcing large institutions like Lehman Brothers into liquidation.

The 2023 Silicon Valley Bank Run

SVB’s failure in March 2023 showed how quickly modern bank runs move. The bank had invested heavily in long-duration Treasury and mortgage-backed securities. When the Federal Reserve began raising interest rates in 2022, the value of these securities fell sharply (because bonds fall in value when rates rise). SVB’s management knew the losses existed, but they were “unrealized”—the securities hadn’t been sold.

On March 8, SVB announced it would sell a large portion of its securities portfolio at a loss to raise capital. Within days, the bank’s uninsured depositors—venture capital funds, fintech companies, and other tech-industry clients—initiated a coordinated withdrawal. Depositors used mobile banking apps to move millions of dollars within minutes. The bank’s cash reserves were depleted in a matter of days. By March 10, SVB was closed by the Federal Deposit Insurance Corporation.

The run was enabled by technology—depositors could execute large transfers instantly—but the mechanics were ancient. Uninsured depositors perceived a threat, believed others would also withdraw, and withdrew immediately.

Prevention and Backstops

Modern financial systems attempt to prevent runs through three mechanisms:

Deposit insurance ensures that small depositors’ money is protected, removing the incentive to panic. This works only if the insurance limit is credible and if depositors believe the insurance fund is solvent.

Central bank liquidity facilities allow solvent banks to borrow against collateral overnight, removing the need to liquidate assets in a fire sale. The Federal Reserve’s “discount window” serves this function, as do overseas equivalents.

Regulatory oversight and capital requirements aim to ensure that banks don’t load up on hidden tail risks. But regulators can’t predict every shock, and they often have perverse incentives to be lenient on large, politically important banks.

The tension is permanent: deposit insurance and central bank backstops reduce the likelihood of runs but may increase moral hazard, since banks know they can take risks and be bailed out. Tighter regulation reduces hidden risks but imposes costs on banks and the real economy. No system has permanently solved this trade-off.

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