Leverage cross‐sections more than a few years apart differ markedly, with similarities evaporating as the time between them lengthens. Many firms have high and low leverage at different times, but few keep debt‐to‐assets ratios consistently above 0.500. Capital structure stability is the exception, not the rule, occurs primarily at low leverage, and is virtually always temporary, with many firms abandoning low leverage during the post‐war boom. Industry‐median leverage varies widely over time. Target‐leverage models that place little or no weight on maintaining a particular ratio do a good job replicating the substantial instability of the actual leverage cross‐section.

Liquidity production is a central function of banks. High leverage is optimal for banks in a model that has just enough frictions for banks to have a meaningful role in liquid-claim production. The model has a market premium for (socially valuable) safe/liquid debt, but no taxes or other traditional motives to lever up. Because only safe debt commands a liquidity premium, banks with risky assets use risk management to maximize their capacity to include such debt in the capital structure. The model can explain why banks have higher leverage than most operating firms, why risk management is central to banks׳ operating policies, why bank leverage increased over the last 150 years or so, and why leverage limits for regulated banks impede their ability to compete with unregulated shadow banks.