Why does your position size grow after a good run without you deciding it should?

Why this matters. Nobody decides to take more risk. The size drifts, one reasonable increment at a time, and each increment is defensible on its own.

Three good outcomes in a row, and the next position is larger. Not doubled — larger. And if asked, you would give a reason that sounds like judgement rather than momentum.

Where the drift comes from

Recent gains are held differently from money that has been in the account a while. Replicated: Thaler and Johnson (1990) showed that a prior gain changes subsequent risk-taking, and that a prior loss changes it differently — the effect runs through how the money is mentally accounted for, not through how much of it there is.

The consequence in practice: money made this week does not feel like capital yet. It feels like room.

And the population-level cost of the resulting turnover is not ours to teach — Portfolio Management covers it in Overconfidence and overtrading.

Why it is invisible from inside

Because there is no moment of decision to catch. There is no point at which you chose to increase risk; there is a sequence of positions, each sized slightly differently for a locally sensible reason. Drift has no event.

Things without events cannot be noticed by attention. They can only be noticed by measurement.

The artefact

A size series — position size recorded per entry, in whatever unit you actually use, in one column, in order. Nothing more.

Plot it or read it. Drift is invisible in the moment and obvious in a column of twenty numbers. The arithmetic of what any given size does to your exposure belongs to What sizing decides — here the point is only that the series exists and that you look at it.

Try it now

Write your last ten position sizes in a single column, oldest first. Do not analyse them. Just look at whether the column has a shape.