Externalities can have a powerful effect on financial stability. This column studies the amplification effect that can operate despite value at risk regulation, which suffers from the ‘fallacy of composition’. It shows that the magnitudes of booms and busts are amplified by two significant externalities triggered by aggregate shocks: the endogeneity of bank equity due to mark-to-market accounting and of bank liquidity due to 'fire-sales' of securitised assets. In addition to economic models, legal and political factors should also be considered.