Payment for order flow is how “commission-free” trading got free: your broker routes your orders to a market-making firm, and that firm pays the broker for the privilege. Whether that’s a clever subsidy or a quiet conflict is one of the longest-running fights in market regulation — here’s what’s actually happening under your buy button.
The mechanics
When you tap buy, retail brokers typically don’t send the order to an exchange. They route it to wholesale market makers who execute it themselves, profiting from the bid-ask spread — and who pay brokers (fractions of a cent per share, billions in aggregate) for that retail flow, which is valuable precisely because small orders are uninformed and safe to trade against. Your trade is usually executed at or slightly better than the public quote (“price improvement”), the broker gets paid, and no commission line appears.
The genuine controversy
Defenders note retail execution is measurably decent and free trading democratized access. Critics note the conflict: the broker’s customer is partly the market maker, and routing can optimize for payment rather than best possible execution — the gap between “at the quote” and “the best achievable price” is invisible to you and worth billions in total. Regulators have circled for years (execution-quality disclosure rules exist; a US ban was debated and not adopted, while the EU moved to phase PFOF out), and enforcement has teeth when disclosure fails — Robinhood’s $65M SEC settlement in 2020 over misleading PFOF disclosure is the standing example.
What you can actually do
Know your broker’s model: it’s in the disclosures (Rule 606 reports name the venues and payments; the ADV or its equivalent tells the story in prose — reading guide). Platforms wear it differently: some earn heavily from PFOF, some renounced it and monetize elsewhere (Public’s report covers the loudest example), and “free” is never free — it’s financed (Loyal3 is what happens when it isn’t). For long-term index investors making occasional trades, PFOF costs are genuinely marginal; for frequent traders, execution quality compounds like a fee. Either way: trace the money once, then trade with your eyes open.