The behaviour gap

Chapter 9 of the mutual funds subject noted that a fund's published return assumes one lump sum held throughout, and that your own money-weighted return can be far below it. This chapter is why that gap exists and what closes it.

Where the gap comes from

A fund's published return is time-weighted: what one rupee invested at the start and never touched would have done.

Your return is money-weighted: it depends on how much you had invested at each moment.

If you invested more after good years and less after bad ones — which is what people do — then more of your money was present for the disappointing periods and less for the recoveries. The fund did what it did; you received something worse, and the difference is entirely timing.

Nothing in the arithmetic requires this gap to be negative. It is negative in practice because the timing errors run in a consistent direction, and the direction is predictable.

The four mechanisms

Buying after good performance. A fund up 40% attracts money precisely because it is up 40%. Chapter 9 of the mutual funds subject: a point-to-point return is largely about its start date, so the number that attracted the money is partly the thing that will not repeat.

Selling after bad performance. The mirror, and more damaging. Selling converts chapter 1's temporary fall into a permanent loss, and it happens at the moment of maximum discomfort, which is near the bottom by definition.

Abandoning a plan too early. A strategy judged over eight months is being judged by noise. Changing it restarts the clock and locks in whatever the noise did.

Chasing the last winner. Moving from a lagging fund to a leading one, repeatedly. Each move pays costs, realises tax, and buys whatever has just run.

Why it is not a willpower problem

Worth saying, because framing it as discipline leads to the wrong fixes.

The impulse to sell during a 35% fall is not irrational in the moment. The information available is that the value has dropped a third, there is no indication when it stops, and every source of commentary is explaining why it will continue. Acting on that is a reasonable response to a bad situation.

What fixes it is not resolve. It is arranging things so the decision does not arise — which is a structural problem with structural solutions.

What actually works

Five, in order of effectiveness.

An emergency fund. Chapter 4. Most forced selling is not panic; it is needing money. Cash removes the mechanism entirely.

A horizon-matched allocation. Chapter 12 of the fixed income subject. If nothing you own has to be sold in the next five years, a fall is an event you read about rather than one you act on.

Automation. A SIP invests on a date regardless of how the month felt. SEBI's own description of the arithmetic — more units when the price falls, fewer when it rises — is the benefit; removing the monthly decision is the larger one.

Something written down. Chapter 11 of the mutual funds subject and chapter 22 of company analysis: a page written while calm, saying what this is for and what would make you sell. During a fall you read it rather than deciding afresh.

Looking less often. Checking a portfolio daily means seeing many more declines than checking quarterly, because short periods are noisier. The portfolio is identical; the experience is not, and the experience is what produces the decision.

The bond allocation argument, again

Chapter 12 of the fixed income subject made it and it belongs here as the conclusion.

If holding bonds is what lets you keep your equities through a severe fall, the bonds have earned far more than their own modest return. The value they add is the equity return you did not forfeit.

Which means the "optimal" allocation on a spreadsheet is not optimal if you cannot hold it. The best portfolio is the one you will actually keep, and that is a statement about you rather than about the assets.

Measuring your own

The only honest version of this chapter's lesson is your own number.

Take your contributions and withdrawals with dates, compute the XIRR, and compare it with the fund's published return over the same span. The difference is yours, caused by your timing, and it is usually larger than any fee you have been worrying about.

Most people have never computed it. It takes twenty minutes and it is the most informative number in their financial life.

The point

A fund's return is time-weighted and yours is money-weighted, so buying after good years and selling after bad ones leaves you behind the fund you held. It is a structural problem, not a willpower one — fixed by an emergency fund, a horizon-matched allocation, automation, a written plan and looking less often.