Across six bubble metrics, the current period has most in common with the GFC era: liquidity has become abundant and investor borrowing has built up, meaning that a sharp downturn would trigger margin calls, amplifying the fall On investor over-optimism, dot-com is a closer parallel, pointing to the risk of sharp equity falls if the bubble bursts Some reassurance comes from improving US corporate health: both average default probability and the share of firms with a default probability above 10%