But some critics, like Fed Governor Jeremy Stein, argue that macro-pru policies to control credit and leverage – such as limits on loan-to-value ratios for mortgages, bigger capital buffers for banks that extend risky loans, and tighter underwriting standards – may not work. Not only are they untested, but restricting leverage in some parts of the banking system would merely cause the liquidity from zero rates to flow to other parts of it, while trying to restrict leverage entirely would simply drive the liquidity into the less-regulated shadow banking system.