Diversification vs. concentration

The most decorated idea in modern finance says: spread your money out, never bet it all on one outcome. The most famous investor alive built his fortune doing roughly the opposite, and has said so, out loud, for decades.

Both of them are right. That is what makes this the most confusing argument a beginner will meet. Every forum thread about it ends the same way: one camp calling the other reckless, the other camp calling the first mediocre, and a reader in the middle who just wanted to know how many things to own.

This article takes both sides seriously, gives each its strongest case, names the people the ideas actually come from, and then does what the forum threads never do: tells you honestly which camp you are in. Spoiler, and this is said with respect: it is almost certainly the first one.

The case for spreading out

Diversification is the practice of not putting everything on one outcome. Own many businesses instead of one, more than one type of asset instead of just stocks, so that no single failure can take you down. The logic is old, but the modern version has a name attached: Harry Markowitz, whose portfolio theory won a Nobel prize, and in whose spirit finance repeats its favourite line, that diversification is the only free lunch in investing.

Free lunch means something specific here. Almost everything in investing trades one good thing for another: more expected reward, more risk. Sensible diversification is the rare exception, because combining holdings that do not all fail together can lower the violence of your swings without lowering your expected return by the same amount. You give up almost nothing and your worst days get less bad. That deal exists almost nowhere else, which is why the phrase stuck.

There is also a humbler, stronger argument, the one this series made in Stock picking: the honest version. To concentrate well, you need an edge: some real, checkable reason you understand a business better than the people trading against you. Most investors, including most enthusiastic and intelligent ones, do not have one. For anyone without an edge, spreading widely, most simply through a broad index fund, is not a compromise. It is the correct move, the way not playing poker against professionals is the correct move. Diversification is the default precisely because it does not require you to be special, and no honest plan should require you to be special.

Diworsification: when spreading out goes wrong

Now the contrarian case, and it starts with getting a word right. The term "diworsification" was coined by Peter Lynch in his book One Up on Wall Street. It is often misattributed to Charlie Munger, who said plenty of sharp things about overdiversification, but the coinage is Lynch's. He originally aimed it at companies: businesses that take the profits from the thing they are great at and buy businesses they do not understand, getting bigger and worse at the same time. Investors adopted the word because portfolios do exactly the same thing.

Here is the portfolio version. You own five businesses you understand. Then you add a sixth because an article praised it, a seventh because a friend owns it, an eighth because it felt risky to have only seven. Repeat for a few years and you hold fifty positions: your best ideas diluted to 2% each, your winners unable to move the needle, and a long tail of names you could not explain if asked. That is not safety. Fifty holdings you cannot explain is fifty ways to be surprised, and you have swapped the risk you could see, one business failing, for one you cannot: owning expensive mediocrity on autopilot.

There is a fund-shaped version of the same mistake, which we met in Index funds and the spectrum of investing. Owning three different funds feels three times as diversified, but if each one is market-weighted toward the same giant companies, you own the same dozen giants three times through different wrappers. The label count went up; the diversification did not. Diversification is measured by how differently your holdings behave, not by how many lines your account statement has.

So the contrarian case is not "concentration is better". It is sharper than that: past a point, adding holdings stops reducing risk and starts reducing only your understanding. Owning more things than you can know is not spreading risk. It is spreading attention.

Protection against ignorance

Warren Buffett's framing, quoted here in spirit and widely on record, ties the two cases together: diversification is protection against ignorance, and makes little sense for those who know what they are doing.

Read it carefully, because both halves are load-bearing and most people only quote the half that flatters them. The second half says concentration can be rational: if you genuinely know what you are doing, spreading into your fortieth-best idea mostly waters down your first. But the first half is not an insult. Ignorance here just means not having done the work, and by that standard nearly everyone is ignorant about nearly every business on earth, including professionals. Protection against ignorance is protection almost all of us need almost all the time.

The operative word in the whole quote is "know". In Stock picking: the honest version we called it the circle of competence: the small set of businesses you can actually explain, whose numbers you have read, whose failure modes you could describe before they happen. Knowing is expensive. It is read filings, follow the industry, do the arithmetic expensive. Feeling confident after a good year is not knowing; it is the market being generous while it lets go of your hand.

Most people, asked that question in private, do not know. Which is why the default stands: wide, cheap, boring. Everything from here to the verdict is the machinery for doing the default well.

Asset allocation: the split does the heavy lifting

Asset allocation is your money's split between the big categories: stocks, bonds, cash. It is the least glamorous decision you will make and the most consequential, because the split, far more than any individual pick, determines how your portfolio behaves. Stocks provide the long-run growth and the stomach-dropping falls; bonds, loans to governments and companies, pay less and swing less; cash goes nowhere slowly, which some years is a feature.

Watch the split do its work in a crash year. Assume stocks fall 30% while bonds eke out 2%, a plausible bad year rather than a prediction, applied to a €10,000 portfolio:

Same crash, three very different nights of sleep. Nothing in that table came from skill; it is pure arithmetic on the split. Your right split depends on when you need the money and how much fall you can watch without panicking, and the honest way to learn the second number is to live through a fall, which is one more argument for starting small and early.

Dollar-cost averaging: a schedule instead of a guess

Dollar-cost averaging means investing a fixed amount on a fixed schedule, say €100 every month, regardless of what prices are doing. It sounds like nothing. Its quiet trick is that a fixed amount buys more units when prices are low and fewer when they are high, automatically, with no forecasting.

Six months of €100 into a fund whose price wobbles:

You invested €600 and hold 75 units, an average cost of €8.00 per unit, even though the average price over those months was €8.50. The month you least wanted to buy, month three, is the month that did you the most good: €100 bought twenty units. At the final €10 price your €600 is worth €750, not because you timed anything but because the schedule made you a systematic buyer of the dip you were too scared to buy deliberately.

Be honest about what this is not. If prices had only risen, investing everything at the start would have done better; averaging in is not a machine for extra returns. Its real product is behavioural: it removes the decision, and removed decisions cannot be flinched.

Rebalancing: the yearly reset

Rebalancing is restoring your chosen split after the market has bent it. Say you chose 60/40 with €10,000: €6,000 stocks, €4,000 bonds. A strong year sends stocks up 25% while bonds gain 2%. You now hold €7,500 and €4,080: a total of €11,580, split 65/35. Nothing failed, but you are now carrying more risk than you chose. Rebalancing sells €552 of stocks, moving you back to €6,948 and €4,632, and it is done for the year.

Notice what that mechanically forced you to do: sell some of what just rose, buy more of what lagged. Sell high, buy low, executed by a calendar instead of by courage. If the next year is the bad one, stocks down 20%, bonds up 2%, the rebalanced portfolio ends at €10,283 versus €10,162 for the drifted one. The €121 is nice but it is not the point, and some years the drifted portfolio wins the comparison. The point is that you went into the bad year holding the risk you chose, not the risk a bull market chose for you. Once a year is plenty; more than that is fiddling.

The honest verdict

So: diversify or concentrate? Here is the verdict this series owes you, with no thumb on the scale.

Concentration multiplies both tails. It makes the great outcomes greater and the terrible ones more terrible, mechanically, with no opinion about which you will get. Hold five equal positions and one goes to zero, you lose 20% of everything; one becomes a ten-bagger, Lynch's word for a tenfold winner, and it nearly triples your whole portfolio. Hold fifty and the same events are a 2% scratch and an 18% boost. Same events, opposite experiences: wide spreading mutes catastrophe and brilliance alike.

Concentration therefore suits a specific, rare person: someone with a genuine, earned edge inside a real circle of competence, the temperament to watch a large position halve without breaking, and finances that survive being wrong anyway. Such people exist. Buffett is one. The uncomfortable statistics from the stock-picking article suggest they are far rarer than the number of people who believe they are one, and the belief itself proves nothing: the confident and the correct feel identical from the inside.

So this is a real choice, and you are an adult who can make it with open eyes. But make it in the right order. Diversification is the default because it is the strategy that works when you are wrong about yourself, and everyone should assume they might be. Earn your way out of it one position at a time, with businesses you can actually explain, and only after the boring machinery, a sensible split, a schedule, a yearly reset, is running underneath you. If one day you can explain a handful of businesses the way their owners could, reading their statements and scorecards for yourself, the kind of work tenbagger, the workspace this blog is part of, was built for, then concentration stops being a gamble and becomes a judgment. Until then, the free lunch is on the table. Eat it.

Do this now

Open your account, or your list of holdings if it lives on paper, and try to write one sentence per holding: what it is and why you own it. No looking anything up. Every holding that gets a real sentence is fine either way. Every holding that gets "it was going up" or a blank stare is your diworsification, found in fifteen minutes. You do not have to sell anything this week. You just have to know which of your holdings you actually chose.

Next in the series: Why you sabotage yourself, on the wiring that makes you buy high, sell low, and feel smart doing both.