Regulators want big, complex banks to hold larger buffers of capital to protect the financial system. Big banks argue this is unnecessary because risk is diversified across their larger balance sheets. Who is right? Natural sciences – especially epidemiology, ecology and genetics – provide clues.

Are big banks?less prone to failure? The traditional economics of diversification suggest so. By scaling up balance sheets across different classes of asset, risks to portfolios will tend, on average, to cancel each other out. Aggregate balance sheet risk is dampened the bigger the balance sheet. Big banks thus benefit from a law of large numbers.

Complex systems – those found in nature, but also in finance – tell a different tale. Here, scaling up risks may cause them to cascade rather than cancel out. The bigger and more complex the structure, the greater this risk.