Concentration is how fortunes are made and it is how they are lost, and the two facts are the same fact viewed at different times.
10 min read · Level 1
Diversification is often described as the price of admitting you do not know which one will work, and that framing quietly insults the people it should protect. The sharper reason to care about position size is that concentration and skill are independent: a concentrated portfolio does not make you more right, it makes your rightness matter more, in both directions. So the question is never whether you are confident - you always are, or you would not be buying - but whether a normal, unremarkable, nobody-did-anything-wrong decline in this one thing would take away years you cannot get back. If the honest answer is yes, the position is too big regardless of how good the idea is, and the idea being good is exactly what will stop you from noticing.
Concentration is having a large share of your wealth depend on one outcome - one company, one property, one sector, one currency, one counterparty. It is worth being precise about the unit, because concentration hides: twelve different technology shares is one bet, not twelve, and a portfolio of local property plus a job in local construction is one economy, not two positions.
The mathematics of why this hurts is unromantic. A position's effect on you is its weight multiplied by its move. The same disaster is a footnote at 5% and an amputation at 60%, and no amount of being right about the thesis changes that multiplication.
Because concentration is the difference between a mistake and a catastrophe, and it is entirely within your control. You cannot control whether a company fails; you can control how much of your future depends on it not failing.
It also interacts badly with the recovery arithmetic. Losses and gains are not symmetrical: the deeper the hole, the more disproportionate the climb out of it, which is why a concentrated loss can cost years even after the judgement that caused it has been corrected.
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Educational, not personalised financial advice. Nothing in the Investor Academy is a recommendation to buy, sell or hold anything — what is right for you depends on your own circumstances and the rules where you live.
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Two portfolios hold the same unlucky company. In the first it is 60% of the total; in the second it is 10%. The company falls 60%.
The first portfolio loses 0.60 times 0.60, which is 36% of everything. The second loses 0.10 times 0.60, which is 6%.
Now climb back. Recovering from a 36% loss requires the remaining capital to grow by 1 divided by 0.64, which is 1.5625 - a gain of 56.25% simply to return to the starting point. Recovering from the 6% loss requires 1 divided by 0.94, which is about 1.0638, or 6.4%.
So a six-fold difference in position size produced a nine-fold difference in the recovery required. That gap is the entire argument, and it did not depend on being wrong about the company - only on how much was riding on it.
The most expensive version is not choosing a concentrated position at all; it is arriving at one by not selling. A holding that does well becomes a larger share of the portfolio automatically, so a portfolio that started sensibly can end up with 70% in whatever worked best, without a single decision having been taken to bet that heavily on it.
The second is counting only the investment account. Someone whose salary, pension, share awards and largest holding are all the same employer has one position with four names, and it is the position that pays for their groceries.
A third is mistaking familiarity for safety. The company you understand best is very often the one you are already most exposed to through your work, which means the knowledge and the concentration have the same source.
Group everything you own by what would have to go wrong for it to fall together: one company, one sector, one country's economy, one currency, one counterparty. Count your income and your property alongside the investments. The groups, not the line items, are your real positions.
For each group, multiply its share of your total by a severe but historically ordinary decline for that kind of asset. The output is a percentage of everything you have, which is the only unit that matters when deciding whether you could live with it.
Set your own ceiling for any single outcome before you are excited about a specific one, and write down what you will do when a winner grows past it. Deciding this while a position is running is a decision made by the position.
Salary, bonus, share awards, options and pension in one employer concentrate both your income and your capital on the same event. It is worth knowing the total, because it is usually far larger than the part that appears on an investment statement.
Whenever a single holding or theme has grown past the share you would deliberately choose today, and any time you are deciding how much to commit to something you feel strongly about.
Do not read this as a case that concentration is always wrong. Concentration is how most large fortunes were actually built, and a diversification-by-default portfolio held by someone with genuine, specific knowledge of one area may be leaving their only real advantage unused. When concentration is right - and what has to be true for it to be - is its own subject in Diversification and Asset Allocation. This lesson is about knowing the size of the bet, not about refusing to make one.
Several well-known corporate collapses - Enron in 2001 being the most studied - left employees holding large amounts of their employer's stock in retirement accounts at the moment they also lost their jobs. The stock and the salary depended on the same company, so a single failure removed both at once. Regulatory and academic attention to employer-stock concentration in retirement plans increased substantially afterwards.
The exposure that does the most damage is frequently the one that never appeared to be a decision - it arrived as compensation, which makes it feel like income rather than a position.
A common shape in portfolio reviews is an investor who chose sensible weights years ago and now has most of their money in two or three holdings, having never made a decision to do so. Each winner grew, each laggard shrank, and the resulting allocation is a record of what already happened rather than a statement about what they now believe.
Left alone, a portfolio drifts towards whatever has worked - which is the same as saying it drifts towards being a bet on the recent past continuing.
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