We study the consumption-portfolio problem in a setting with capital gain taxes and multiple risky stocks to understand how short selling influences portfolio choice with a shorting-the-box restriction. Our analysis uncovers a novel trading flexibility strategy whereby, to minimize future tax-induced trading costs, the investor optimally shorts one of the stocks (or equivalently, buys put options) even when no stock has an embedded gain. Alternatively, an imperfect form of shorting the box can reduce aggregate equity exposure ex post. Given these two short selling strategies, it is common for an unconstrained investor to short some equity while a constrained investor holds a positive investment in all stocks. With no shorting, the benefit of trading separately in multiple stocks is not economically significant.

We analyze competition and risk management at central counterparties (CCPs) using a granular transaction-level dataset, and find that CCPs decrease collateral in response to lower collateral at their competitors, an effect that becomes stronger as the correlation between positions increases. To interpret our findings, we derive a model in which collateral is driven by risk and CCP competition. Our results are consistent with the model and suggest that a single monopolistic CCP would require more collateral. We also show that amid the substantial increase in collateral during the Covid-19 pandemic, the probability of a margin breach did not significantly change.