When to Sell Stocks: The Only Three Good Reasons

Your portfolio is deep in the red. You have opened the app for the third time today, and your thumb is hovering over the sell button.

Everyone teaches you what to buy. Almost nobody teaches you when to let go, so the purchase gets researched and slept on, while the sale happens on a Tuesday evening in about four seconds.

Three reasons justify a sale for a long-term investor. Every one of them gets written down before the bad day arrives, and none of them begins with a number on a screen.

What selling in the storm actually costs

The expensive mistake in investing is rarely the purchase. It is a reasonable portfolio sold at an unreasonable hour — and the reason it costs so much is that the market does its best work immediately after doing its worst.

J.P. Morgan Asset Management publishes the calculation every year. Take $10,000 in the S&P 500 Total Return Index from 2 January 2006 to 31 December 2025. Left alone, it becomes $80,619, or 11.0% a year. Miss only the ten best days of those twenty years and you are left with $35,866, or 6.6%.

Now push it a little further, which is the part that stops people.

S&P 500 Total Return · $10,000 · 2 January 2006 to 31 December 2025

Miss 40 days out of roughly 5,000, and twenty years pay you nothing

Annualised return, and what $10,000 turned into. The bars do not just get shorter. Between “missed 30” and “missed 40” they change sign.

Miss forty days out of roughly five thousand and the twenty years pay you minus 0.3% a year. Not a smaller gain. A loss: $9,462 against the $10,000 that went in.

Nobody misses forty days by accident, of course. You miss them by being out of the market during the weeks when everything happens at once, which is exactly what a panicked sale arranges for you: six of the ten best days fell within two weeks of the ten worst, and five of those six came after the worst days. The second-worst day of 2020 was 12 March. The second-best day of that year was 13 March.

Two honest caveats, because this chart gets waved around a lot. It is a hypothetical calculation, gross of fees, and it is not a forecast of anything. And the figures move with the window: the video quotes the same slide from the previous edition, which ran to the end of 2024 and counted seven of the ten best days near the ten worst rather than six. Same slide, one year further on. A number like this is only true if you name the window it came from.

There is a slower version of the same mistake, and it needs no crash at all. Private investors sell their winners too readily and hold their losers too long — the disposition effect, established by Terrance Odean on brokerage records in 1998. No storm required. The portfolio simply gets pruned in the wrong direction, quietly, over years.

So the problem was never selling. It is where the decision gets made. Institutions make it somewhere else entirely, which brings us to the three reasons — and to the one that does most of the work.

Reason one: your band broke

This is the reason nobody puts in a video thumbnail, and it is the one that will actually make you sell something this decade.

Start with what happens if you do nothing. Suppose you chose 70% equities and 30% bonds, because that split matched the risk you could live with. Equities then double while bonds go nowhere. Your 70 becomes 140, your 30 stays 30, and the portfolio is now 82% equities. Nobody decided that. It is arithmetic, and the arithmetic runs in one direction: left alone, every portfolio drifts toward whatever performed best. You end up holding the risk profile of a bull market you have already had.

Rebalancing is the correction. The institutional part is not that it happens but how it is triggered.

Most people are taught the calendar version of this, which is to rebalance every January, or every quarter, whatever the portfolio has actually done in between. It works, and it has a flaw — the date knows nothing about the portfolio. It fires in a year when nothing moved, and it sits on its hands through a March in which everything did.

The professional version puts the trigger on the portfolio instead. You set a band around your target (five percentage points either side is a common private-investor width) and you check at fixed intervals, monthly or quarterly. Then you do nothing at all, for months on end, unless the band actually breaks.

Threshold rebalancing · target 70% equities · band 5 points either side

The rule does nothing for months, then does one thing

Each dot is a scheduled check. Inside the shaded corridor you look and close the app. The band only breaks when one side has run and the other has lagged, which is why the trade it forces is always the uncomfortable one.

This is not a retail simplification of what institutions do. It is what they do. Norway’s Government Pension Fund Global runs a strategic equity share of 70%, and rebalances when the equity share in its benchmark deviates by more than 2 percentage points from that weight, measured on the last trading day of the month. A target, a band, a fixed check date. The band is tighter than a private investor needs, because the fund trades at a cost almost nobody else has.

Look at what the rule quietly arranges. Your band can only break when one side has run hard and the other has lagged. So the thing you trim is the thing that got expensive, and the thing you top up is the thing nobody wants. You are selling high and buying low, not because you predicted anything, but because a rule you wrote in calm weather ordered you to on a day your gut was screaming the opposite. It is the disposition effect, inverted, and put on rails.

This is not a simplified retail version of an institutional practice. It is the institutional practice. Norway’s Government Pension Fund Global holds 21,268 billion kroner, 71.3% of it in listed equities at the end of 2025. It runs a strategic equity share of 70% and rebalances when the benchmark’s equity share deviates by more than 2 percentage points from that weight, measured on the last trading day of the month. A target, a band, a fixed check date, and nothing in between. The band is tighter than yours should be, for the unromantic reason that the fund trades at a cost you cannot match.

Now the part I am not going to give you.

I am not going to tell you how much extra return rebalancing earns, because there is no honest single number. It depends on which assets and which window — and in a long one-way bull run, a rebalanced portfolio can lag one that was simply left alone. Anyone quoting you a precise figure has picked a period. What the rule reliably does is keep the portfolio at the allocation you actually chose, which is a risk statement, not a return promise. Any return effect is a small and uncertain by-product of the discipline.

One practical note before the other two reasons, because it saves people money. Where you are still paying money in, you can often rebalance with new contributions alone — direct them at the underweight side until the band closes. No sale, no transaction, and no disposal to be taxed. Tax treatment differs by country and by situation, so before any large sale it is worth checking rather than assuming. That sentence is deliberately vague; the alternative would be advice, and this is not that.

The band, the target and the check date belong on a written page. That page has a name in institutional settings, the investment policy statement, and the portfolio governance piece covers what else belongs on it. If you want the definition on its own, rebalancing has an entry.

Reason two: your life changed

The money was for something. When that something moves closer, a house in three years or retirement coming into view, the risk you can afford drops with it, and selling down risk is your plan executing itself rather than your nerve failing.

That is the whole logic of how much of your money belongs in stocks, read from the other end.

One distinction matters here, and it is what your emergency fund is for. A life event you can see coming is a reason to sell. A surprise bill is not — that is the buffer’s job, and a portfolio forced to liquidate into a bad week is precisely the outcome the buffer exists to prevent.

Reason three: your thesis broke

This one applies only if you own individual companies, and it is the one people get most wrong.

If you bought a company for a reason — its market position, or the fact that its customers cannot easily leave — then the day that reason stops being true, the position has lost its job. That is a sale.

The price is not the thesis. A share can fall 30% while the reason you bought it is entirely intact, and it can print a record high while that reason quietly dies. The chart tells you what other people currently think. Your thesis is what you think, and only one of those is a reason to act.

That is the short version. The long one is when to sell an individual stock: kill criteria, thesis creep, and a company autopsy that shows how slowly a broken thesis announces itself.

The test for everything else

Which leaves a clean test for every other impulse.

The market feels high? Not one of the three. Notice that “feels high” is a feeling, while a breached band is a rule you set on a calm day. A headline says crash? Not one of the three. Your stomach hurts when you open the app? Understandable, and still not one of the three.

If none of the three applies and the only argument is today’s price or today’s fear, then the thing you are about to do is not selling. It is market timing with extra steps, and the chart at the top of this article is what that costs.

The cost is measurable, incidentally, and I have measured it. Rebuilding investor returns from 341,049 SEC filings put the gap between US fund investors and their own funds at +0.36 percentage points a year for 2020 to 2025, with a range of −0.06 to +0.62 depending on how the benchmark is defined. That is far below the multi-point figures the industry likes to quote, and it is still a gap, and it is made of decisions taken at moments like the one your thumb is in right now. If you want to see what your own version of it looks like, the behaviour gap simulator will show you.

The page you write in calm weather

An institution does not rely on willpower for any of this. It writes its selling rules into a mandate before a single euro is invested, and then the rules are simply what happens.

You need something much smaller. One page, and fifteen minutes.

Fifteen minutes, once, in calm weather

The page you read instead of the position

My selling plan

Written on a quiet day. Read on the loud ones. Not changed on the loud ones.

1

Rebalancing

Target 70% equities / 30% bonds, band 5 points either side. I check on the last Sunday of the month. I act only when the band actually breaks, and only back to target.

2

Timeline

I do not need this money before 2041. When that date moves inside ten years, the equity share comes down — not because of a price, because of a date.

3

Thesis, one line per individual stock

"I own this because its distribution network is the reason customers stay." If that sentence stops being true, the position has lost its job and I sell it.

Not on this page

• ✕ The number on the screen today
• ✕ A headline about a crash
• ✕ The feeling that the market is high
• ✕ A bad evening

If none of the three lines applies, there is nothing to decide. Close the app.

Line one is your rebalancing rule: a target split, a band, and a fixed date to look. Line two is your timeline: when you genuinely need this money, and whether today’s risk level still fits that date. Line three is one sentence for every individual stock you own, beginning “I own this because”. If you cannot finish that sentence, you have learned something useful before you have written anything down.

Then one habit. It is the only part of this that asks anything of you on the night itself. When your thumb hovers over the sell button, open the page instead of the position. If one of your three reasons is on it, sell calmly. If not, close the app.

That Tuesday evening will come again — the red screen, the itchy thumb, the certainty that this time is different. What changes is that the decision will already have been made, months earlier, by a calmer version of you who had the whole weekend to think about it.