Some HFTs stopped supplying liquidity during flash crashes, others didn&#x27;t. The ones who didn&#x27;t were actually the ones to end the flash crash; they see great bargains and buy them up.<p>The main reason HFTs pull out of markets during flash crashes and similar events is regulatory (broken trades). The only HFTs who can stay in the market during a flash crash are the ones who believe they can predict the retroactive actions of human regulators. Obviously this will only be a small subset of HFTs.<p>If you want to change this, eliminate the breaking of &quot;clearly erroneous trades&quot;. (Japan doesn&#x27;t have this rule and they do fine.)