What "risk" really means

Risk is the probability the money doesn't come back. A price that moves a lot is something else, and it has its own name: volatility.

An asset is anything you buy expecting it to be worth more later: a share, a bond. One that swings hard but ends up multiplying in value has been highly volatile and barely risky in the sense that counts. Confusing the two is the mistake that costs the most.

Why more expected return demands more risk

Because nobody pays more for taking on less. If an investment existed with low risk and a high return — what it hands back for every euro you put in — everyone would want to buy it.

And buying pushes the price up, while what the investment hands out stays where it was. Picture something that pays out €5 a year. At a price of €50, that €5 is 10% of what you paid. If the price climbs to €100, the same €5 is 5%. The price stops rising when the return no longer compensates for the risk, and by then the bargain is gone.

Anyone promising otherwise either hasn't done the math or isn't being honest.

The figures that lay out the map

On the series Aswath Damodaran, a professor at New York University, has published going back to 1928 and updated in January 2026 with data through 2025, the US stock market has returned 10.0% a year compounded before inflation. Mid-grade corporate bonds — the Baa rating from Moody's, one of the agencies that score how likely a borrower is to pay back what it owes — 6.6%. Cash, meaning three-month Treasury bills, 3.4%.

Compounded is the steady rate that, stacking one year on the last, gets from the first figure to the final one; it is not the average of the separate years.

That gap between what stocks pay and what cash pays has a name, the risk premium, and a specific reason: stocks are far more uncertain in the short run, and the market compensates for tolerating that uncertainty.

The trap of looking only at the average

That 10.0% is a near-century average, and it hides enormous variability: there are decades where stocks barely beat inflation and decades where they beat it comfortably.

That average tells you nothing about the next ten years. Anyone using it as a promise is doing exactly what this course tries to avoid.

What to do with this relationship

What you pick is the level of risk you can sustain. The return follows, and it won't always be the highest one on the table.

And "sustain" has two sides, one about money and one about nerves, because the profile sheet at the end of this chapter asks about them separately:

• Risk capacity: how much you can lose without your life changing. It's arithmetic: it depends on your cushion, your horizon, and your income.
• Risk tolerance: how much you can lose without losing sleep. It's emotional, and you discover it in your first real drawdown, not by filling in a questionnaire.

The lower of the two governs. Financial capacity to sit through a 30% drop is worthless if you sell in week three because you can't stand it.

How volatility differs from permanent loss

Volatility measures how much a price moves, up and down. Risk measures something else: the probability that money never comes back. They're related, but they aren't the same.

How it's measured, so it isn't a vague word

Volatility is calculated using the standard deviation of returns. Standard deviation is a number that says how spread out the figures are: if every year gives roughly the same, it comes out small; if some years give a lot and others very little, it comes out big.

If an asset has 20% annual volatility, roughly two years in three its return will land within a band of twenty points above or below its average. An asset with 8% volatility moves within a much narrower band.

That number says nothing about direction. A highly volatile asset can rise a lot or fall a lot; volatility only tells you the typical size of the move, not which way.

When we reach the chapter on charts you'll see another way to measure the same thing, the ATR (average true range), built for day-to-day rather than yearly figures. Same idea, different unit.

The two examples that separate them

Volatile and barely risky. A stock that swings 5% either way each week but has multiplied in value over ten years. Whoever held it, won. The movement was enormous; the real risk, in the sense of money not coming back, was low.

Barely volatile and highly risky. A company whose shares hardly move for months and then goes bankrupt, shares at zero. Prior volatility was low and real risk was total.

The gap this creates. To know whether a move is normal for an asset or a signal of something, you need to know how much that asset normally moves. And that isn't shown anywhere: your app displays today's price, not whether that −4% is an ordinary Tuesday or something exceptional for that company.

It's one of the things Volatly calculates for each asset separately: its own volatility profile, rather than one rule applied to everything.

A −4% in a company that moves 1% a day and a −4% in one that moves 5% a day are two completely different pieces of news, and without that reference both look identical on screen.

When a paper loss becomes real

A price drop is an unrealised loss as long as you don't sell. It becomes a real loss at the exact moment you sell below what you paid. That distinction separates a temporary drop from permanent damage.

Why this isn't a convenient excuse

"You haven't lost until you sell" sometimes gets used to avoid admitting a mistake.

The honest version is this: today's price is the best available estimate of what your position is worth today. Your position is what you hold in one particular asset. If you're down 30%, you're down 30%, and saying otherwise is self-deception.

It is true that the loss can still reverse if the company keeps working and the price recovers. Once you sell, you eliminate any possibility of recovery.

The mistake this produces in both directions

Panic-selling during a normal drop turns volatility into permanent loss. It's the most common error and the most expensive.

But there's an opposite one, equally costly: holding a position indefinitely when the company underneath is no longer the same one, just to avoid admitting the loss. If the reason you bought no longer holds, waiting is refusing to look.

The two situations differ on one thing: whether that reason, what investors call the thesis, still stands. That requires having written it down somewhere before buying, which is what the logbook in the chapter on method is for.

How much diversification you need and when it stops helping

Diversifying means spreading money across assets that don't move together. It works very well up to somewhere between thirty and fifty well-chosen holdings, and barely adds anything after that. And it only removes one kind of risk: nobody avoids the other.

The two risks, and which one you can get rid of

Specific risk is what affects one company: bankruptcy, a lost lawsuit, a flagship product failing. Diversifying reduces it substantially, because it's unlikely that twenty different companies hit trouble in the same week.

Systematic risk is what hits the whole market at once: a recession, a broad rise in interest rates, a pandemic. Nobody eliminates this. It's the price of being invested, and it's why the market pays the risk premium we saw at the start.

How many holdings, per the evidence

A classic study by Evans and Archer, published in 1968 in the Journal of Finance, an academic journal, found that the benefit of adding holdings runs out fast: past about ten, each new one reduces far less than the last. The figure that gets quoted — "80% of the risk with ten or fifteen holdings" — is a summary that came later, not a conclusion of the study.

More recent work, such as the one Meir Statman published in 1987 in another academic journal, the Journal of Financial and Quantitative Analysis, puts the number considerably higher, in the region of thirty to forty.

And the number has kept climbing. Campbell, Lettau, Malkiel and Xu showed in 2001, in the Journal of Finance, that between 1962 and 1997 each company moved more on its own and less with the market as a whole, which forces you to hold more names for the same effect. Hicham Benjelloun redid Evans and Archer's work in 2010 and landed on forty or fifty.

The nuance that invalidates the number: how they move together

Thirty tech stocks aren't diversified. Ten holdings spread across sectors that react differently to the economic cycle, to the good times and the bad, protect considerably more than thirty that rise and fall together.

Risk comes down when your holdings stop falling for the same reason at the same time. How many of them sit in your portfolio, which is everything you own put together, doesn't do that work on its own. Buying twenty stocks in the same sector gives the feeling of diversification without the effect.

That idea (how much two positions move together) is called correlation, and it has direct consequences for how much money to put behind each trade. It gets developed fully in how much to risk on any single trade, because that's where it changes decisions.

Two risks nobody mentions that affect you today

Beyond the risk of a company doing badly, there are two that show up unannounced and affect most investors in Spain. Neither is solved by diversifying across stocks.

Currency risk, which applies whether you chose it or not

If you buy US stocks from a euro account, you're taking on two risks at once, not one: the company's, and the euro-dollar exchange rate.

Imagine you buy shares in a US company and a year later that stock is up 10% in dollars. Good news.

But if over that same year the dollar has weakened 10% against the euro, your return in euros, the currency you pay rent in, lands near zero. The company did well and you gained nothing.

It works the other way too: a flat stock in dollars with a dollar that strengthens 10% gives you 10% in euros without the company doing anything.

None of this argues against investing outside the eurozone. You just need to know that when you look at a US stock's return in your app, that figure may be in dollars and may not be what you actually take home.

The conversion has a cost too, and we'll put numbers on it in the next chapter.

The risk of your income and your investments falling together

This is the most invisible of all and appears on no risk questionnaire.

If you work at a bank and also hold bank shares, your salary and your portfolio depend on the same thing.

In a financial sector crisis, both go wrong at once: exactly when you might need to sell investments is when they're worth least, and with your job in question too.

The extreme case is holding shares in your own employer, common when they form part of compensation. That's a double concentration: your salary and a chunk of your wealth depend on the same company.

There's no universal rule here, but there is a question worth asking: if my sector does badly, does my portfolio fall too? If the answer is yes, your real diversification is lower than your number of holdings suggests.

Why leverage multiplies losses first

Leverage means trading with more money than you have, borrowing the rest from the broker. It multiplies gains and losses identically, but you don't reach both with the same probability: the loss ejects you from the position and the gain doesn't.

How it works, with numbers

With 10:1 leverage, €1,000 of your money controls a €10,000 position. If the price rises 1%, you make €100, which is 10% on your actual money. If it falls 1%, you lose the same.

The asymmetry shows up at the extreme. With that same 10:1, a 10% fall in the asset wipes out the full €1,000. The asset still exists and may recover next week, but you're no longer in it: your position was closed when the collateral ran out, and the collateral is that €1,000 of yours, the money the broker holds back to cover losses.

Without leverage, that same 10% fall would have cost you €100 and you'd still hold the position to see what happens next. Leverage multiplies the outcome, and it also decides whether you're still in the game.

The margin call

When losses approach wiping out the collateral posted, the broker warns you: either add money, or the position gets closed. The window is usually short, sometimes hours.

Plenty of outdated information circulates about what happens next.

Under the ESMA (European Securities and Markets Authority) and CNMV (Spain's securities regulator) measures, brokers are required to provide negative balance protection to retail clients, that is, to individuals. In the European Union you can no longer end up owing the broker more than you deposited on these products, which used to happen. You can lose everything, but not more.

The legal caps in Spain

Contracts for difference are the CFDs from the opening session.

ESMA set these caps in 2018 for the whole European Union, and the CNMV made them permanent in Spain in 2019. They cap the leverage that can be offered to a retail client on contracts for difference, by asset type:

Asset type	Maximum leverage
Major currency pairs	30:1
Other currencies, gold, and major indices	20:1
Other commodities and non-major indices	10:1
Stocks	5:1
Crypto-assets	2:1

Notice the ordering: the more an asset moves, the less leverage the regulator permits. Crypto-assets, the most volatile on the list, are capped at 2:1. That scale is an official risk classification, drawn up by the supervisor.

There's also a mathematical asymmetry that makes leverage more dangerous than it looks: recovering from a large loss requires a proportionally much larger gain. That relationship has concrete numbers, and gets covered fully in drawdowns, streaks, and sample size.

What to remember

• Volatility is how much a price moves; risk is the probability the money doesn't come back. Not the same thing.
• A loss becomes permanent when you sell, but refusing to look at a broken thesis isn't patience either.
• Diversification stops adding somewhere between thirty and fifty holdings, and does nothing if they all move together.
• Buying abroad adds currency risk, and your own salary may be correlated with your portfolio without you noticing.
• Leverage multiplies the outcome and also decides whether you stay in the game; in the EU you can no longer lose more than you deposited.

Related: what each indicator measures · volatility, leverage, and what the regulator banned · what a stock is and why a company goes public