The psychology of trading · Chapter 5 of 9

Position size is a psychological decision

Q: Which effect of oversizing appears first?

Degraded execution — closing early or freezing — The account damage comes later. The immediate effect is on management: a position too large to sit with changes every decision you make while it is open.

Q: You have six losses in a row with a method that historically wins about half its trades. What does this most likely indicate?

Normal variance, which any method with losses will produce — Runs like this are an ordinary property of sequences with losses, not a signal in themselves. Deciding a method is broken from a short streak is how working methods get discarded.

Foundation3 min read · part of The psychology of trading

Position size is usually taught as arithmetic: risk per trade, stop distance, lots. The arithmetic matters, but the reason it matters is behavioural. Size determines how a normal losing streak feels, and how it feels determines whether you still follow your method at the end of it.

Every method has losing streaks. They are not evidence of anything being wrong — a method that wins slightly more often than not will still produce runs of consecutive losses, and the longer you trade the more certain it becomes that you will meet a long one. The question is not whether it happens but what state you are in when it does.

This is the real argument for small risk per trade. At a modest size, a run of six losses is uncomfortable and survivable, and you are still the person who wrote the plan. At a size that felt exciting, the same run is an emergency, and emergencies produce exactly the behaviour that turns a drawdown into a blown account.

There is a second effect that gets less attention: oversizing degrades execution before it degrades the account. When a position is too large you watch it differently. Small adverse moves become significant. You close early, or you cannot bring yourself to close at all. The size changed the trade you were capable of managing.

Practically: choose a size at which you can be wrong several times in a row and still take the next signal exactly as written. If a normal losing streak at your current size would change how you trade, the size is wrong — regardless of what the arithmetic permits.

What to take away

Losing streaks are certain; your state during them is what size controls.
Oversizing damages execution before it damages the balance.
The right size is the one at which a normal streak does not change how you trade.
The arithmetic sets an upper bound. Your behaviour sets the real one.

This chapter, measured against your own trades

In the app the same chapter ends in your figures rather than an example: how often you did the thing it describes, over your last ninety days. You pick one change to make, and Evidence checks afterwards whether it actually changed — from your journal, arithmetic, no opinion involved. Questions you get wrong come back a week later and again a month after that.

Open the free plan →

Check that it stuck

Answers shown — in the app these are asked before you see them, and the ones you get wrong come back after a week.

What is the strongest practical test for whether your position size is right?

A normal losing streak at that size would not change how you take the next signal — A percentage rule is a useful upper bound but says nothing about you. The behavioural test is the binding one, because the point of sizing is surviving streaks with your method intact.

Which effect of oversizing appears first?

Degraded execution — closing early or freezing — The account damage comes later. The immediate effect is on management: a position too large to sit with changes every decision you make while it is open.

You have six losses in a row with a method that historically wins about half its trades. What does this most likely indicate?

Normal variance, which any method with losses will produce — Runs like this are an ordinary property of sequences with losses, not a signal in themselves. Deciding a method is broken from a short streak is how working methods get discarded.

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