The foundations track explained the order book: two prices, a spread, depth that thins out. This lesson answers the question that book cannot: why does the spread exist at all, and who decided how wide it should be?
The answer is not "the broker's cut". It is something stranger and more useful — the spread is a measure of how afraid the other side is of you.
The Spread Is Priced Fear
Here is a classic model, and it is exact rather than empirical. A stock is worth either 101 or 99, equally likely, so its fair value is 100. Some fraction of traders are informed — they know which. The rest are noise, buying and selling for reasons unrelated to value.
A market maker cannot tell them apart. All they see is an order arriving. But the order itself carries information: a buy is more likely to come from someone who knows the value is 101. So the maker must quote prices that break even given what the order implies. Solve it:
— 0 percent informed: spread 0.0 basis points — 5 percent informed: 10 bps — 10 percent informed: 20 bps — 20 percent informed: 40 bps — 50 percent informed: 100 bps — 100 percent informed: 200 bps — the maker cannot quote at all
Read the first line. If literally nobody knows anything, the spread is zero. There is nothing to protect against, so protection costs nothing. Every basis point of spread you have ever paid exists because somebody, somewhere, might know something you do not.
That reframes the cost entirely. The spread is not a service charge. It is an insurance premium the market maker charges against being picked off — and you pay it whether or not you are the one doing the picking.
What This Means If You Know Nothing
Follow it through, because the conclusion is uncomfortable.
At 20 percent informed flow, the maker quotes 100.20 offered and 99.80 bid. If you are an uninformed buyer, you pay 100.20 for something worth 100. You have handed over 20 basis points above fair value — not because you were unlucky or slow, but because the price is set to survive the informed traders, and you are being charged the same toll as them.
Round trip in and out, knowing nothing: 40 basis points of pure transfer. You did not lose to volatility. You lost to a spread that was calibrated against a threat you never posed.
The informed trader pays that same 20 basis points and takes a full franc of value. Same price, opposite outcome. The spread is a flat tax on participation and a bargain for anyone with real information.
Why Your Order Flow Gets Bought
This explains something that confuses people: why firms pay for retail order flow rather than avoiding it.
Run the model at very low informed fractions. Against flow that is 0 percent informed, the fair spread is 0 bps. At 2 percent, 4 bps. At 5 percent, 10 bps. So if a wholesaler believes a stream of orders is essentially uninformed, it can fill that stream at almost no adverse-selection risk and keep most of whatever spread it captures.
Retail flow is valuable precisely because it is not a threat. You get filled quickly and smoothly not as a courtesy but because your order carries no information anyone fears. The moment retail flow starts predicting the next move, it stops being cheap to fill and starts being avoided — which is exactly what happens to flow that turns out to be informed.
This is worth sitting with. The ease of your execution is evidence about how seriously the market takes your opinion.
The Question To Ask On Every Fill
There is one habit this whole lesson exists to produce. When you get filled instantly at a good price, ask: why was someone happy to take the other side?
Sometimes the answer is benign — they were a maker earning the spread, or a fund rebalancing, or someone whose horizon is different from yours. Those trades are fine; you both got what you wanted.
But if the answer is "because they knew something", you have already lost and the screen has not told you yet. An easy fill is not confirmation. It is, at best, neutral information — and occasionally it is the market politely accepting your donation.
The Catch
The model is deliberately stripped down: two possible values, one trade at a time, no inventory, no competition between makers, no fees. Real spreads also compensate makers for inventory risk and operational cost, so a real spread is wider than pure adverse selection implies. The direction is right; the level is not a quote.
It also assumes the informed traders are actually informed. In reality most people who believe they have information are wrong, which means real flow is less toxic than participants fear and spreads are partly a tax on collective paranoia.
And nothing here says you cannot be the informed one. People with genuine information exist. The point is that the arithmetic charges you as though you might be, and pays you only if you actually are.
Do This
— On your next fill, write one sentence explaining who took the other side and why they were willing. If you cannot, you have learned something about how much you know. — Compare the spread on something liquid and something thin. The gap is the market's estimate of how much dangerous information is floating around each. — Notice when your fills get harder. If a name starts trading away from you before you finish, the book has decided your order carries information — and that is a fact worth more than most indicators.
This is education, not financial advice. It's general information, not a recommendation for your situation, and it doesn't account for your circumstances. Rules and figures change — verify before you act, and speak to a qualified professional for decisions that matter.