Trading volume shares and market quality: Pre- and post- zero commissions

•Retail investors shift assets to brokers eliminating commissions.•Zero-commission brokers increasingly route orders to wholesale market makers.•Investors submit more orders and smaller-sized orders.•Retail investors earn less price improvement per share after zero-commissions.•Effective spreads decline as retail traders post orders within the bid-ask spread and intraday volatility increases. After the adoption of zero-commissions by major brokers, they increasingly route orders to wholesale market makers to possibly earn payment for order flow given the loss of commissions. Retail investors assets held by zero-commission and commission-charging brokers increase 7 % and decrease 9 %, respectively. Retail investors earn less price improvement per share and submit more orders and smaller orders. Effective spreads decline because retail limit prices are increasingly posted within the bid-ask spread. Intraday volatility increases and price impact falls, as orders become more uninformed, while realized spreads remain unchanged.