The fund went up. Its investors went down.

There are two honest ways to measure how an investment did, and they answer different questions. The time-weighted return asks: how did a pound do if it was left in from the first day to the last? That is the number funds advertise, and it is the right way to judge the manager, because it strips out the timing of money coming and going. The dollar-weighted return asks something else: how did the actual money do, given when it actually arrived and left? That is the number investors live.

When those two numbers diverge, it is because money was not present in equal measure across time — and it almost never is. People feed cash into a fund after it has done well and yank it out after it has done badly, so the most dollars are exposed to the disappointing stretches that follow the good ones, and the fewest dollars are present for the recoveries that follow the bad ones. The fund’s return is an average over time; the investor’s return is an average weighted by money present. Chase performance, as the slider does, and that weighting works systematically against you. The gap between the two has a name: the behaviour gap.

The cleanest illustration needs only three years and a steady saver. Suppose you add £1,000 at the start of each year to a fund that gains 10%, gains 10% again, then loses 10%. Judge the fund and it did fine: a pound left in for all three years grew at about 2.9% a year. Judge your money and it did not: by the loss in year three you had roughly £3,000 exposed, against the £1,000 that caught the first year’s gain, so your dollar-weighted return is slightly negative.

Nothing dishonest happened here. No fee was hidden, no return was faked. Two correct calculations simply describe two different things, and the one printed on the brochure is not the one in your account. Now imagine this effect not over a gentle three-year drip but over a decade of fear and greed, with a fund that lurches violently — and you have the real thing.

The behaviour gap is manufactured by a single, deeply human habit: we move money toward what has just gone up and away from what has just gone down. A fund posts a dazzling year, the press notices, the inflows arrive — right at the top. The fund then has a terrible stretch, nerves break, the money leaves — right at the bottom, just before the rebound. Each decision feels prudent in the moment. In aggregate it is a machine for buying high and selling low, over and over.

This is why the gap is a regression-to-the-mean story as much as a behavioural one. Stellar recent performance is partly skill and partly luck, and the luck tends to reverse; piling in right after it is piling in right before the reversion. The crowd’s timing isn’t merely unlucky — it is structurally backwards, chasing exactly the returns least likely to repeat.

This is not a thought experiment. Morningstar has measured the gap across tens of thousands of funds for nearly two decades in its annual Mind the Gap study. Over the ten years to the end of 2024, the average dollar invested in US funds earned about 7.0% a year while the funds themselves returned about 8.2% — a gap of roughly 1.2 percentage points a year, or about a seventh of all the returns on offer, lost to timing alone. And the gap is not spread evenly.

The emblematic case is the CGM Focus Fund. Over the decade from 2000 to 2009 it was the single best-performing US diversified stock fund, run by a feted manager and compounding at about 18% a year while the S&P 500 went nowhere. Yet Morningstar calculated that the average investor in it lost about 11% a year over the same decade — a roughly thirty-point chasm between the fund and its owners. The reason was textbook: after the fund soared 80% in 2007, investors poured in some £2bn-plus, just in time for a 48% crash, then fled near the bottom. The best fund of its decade made its typical shareholder poorer.

Read the right return. A fund’s headline figure is its time-weighted return — the manager’s score, not yours. Where it is available, look for the investor or dollar-weighted return, which is what shareholders actually captured. A large gap between the two is a warning that this fund is hard to hold, however well it performs.

The cure is mostly inaction. The whole gap is made of moves — buying after rises, selling after falls. Automating contributions, rebalancing on a calendar rather than a feeling, and simply doing less are not signs of laziness; they are the mechanism by which ordinary investors capture more of their funds’ returns. The data is blunt about it: the funds people trade least — boring, diversified, all-in-one funds — have the smallest gaps.

Distrust the thrilling fund. The wider a fund’s swings and the louder its recent triumph, the more it invites the timing that destroys returns, and the bigger its historical gap tends to be. A smoother fund you can actually sit through will often leave you richer than a brilliant one you cannot.

So the question to ask of any past return waved in front of you is whose return it is: the fund’s, or its investors’? If the number assumes a pound left untouched from the first day to the last, it is describing a discipline almost no one practised. The rest of the library is full of averages that mislead about other people; this is the one where the average misleads you about the one investor you can actually control. The gap is not the market’s doing. It is ours.