Diversification

There is one trick in investing that genuinely lowers your risk without proportionally lowering your expected returns. It is so well-established that economists call it "the only free lunch in finance." It is also the thing most beginners ignore until it is too late.

It is diversification: spreading your money across many different investments so that no single one can sink you.

Why it works

Imagine you bet your entire $10,000 on one company. If it doubles, great. If it goes bankrupt, you have lost everything. Your range of outcomes is enormous, in both directions.

Now imagine you split that $10,000 across 100 different companies, $100 each. A few will likely go to zero. A few will triple. Most will land somewhere in the middle. Your overall return ends up close to the average of the group, with much smaller swings.

You did not give up much expected return, because the average performance of 100 companies tracks the average of one company in the same sector. But you cut the volatility dramatically because the winners and losers cancel each other out.

What "diversified" actually means

Owning 50 different tech stocks is not really diversified. They all move together. When tech crashes, they all crash. Real diversification means spreading across things that respond differently to the same conditions.

There are several axes you can diversify along. The more you cover, the more bulletproof your portfolio becomes against any single nasty surprise.

• Companies: hundreds, not handfuls. Reduces single-company blow-up risk.
• Sectors: technology, healthcare, finance, energy, consumer goods, etc. When one sector tanks, others often hold up.
• Geographies: US, Europe, emerging markets. A single country having a rough decade does not destroy you.
• Asset classes: stocks, bonds, real estate, commodities, crypto. They respond very differently to inflation, recessions, and crises.
• Time: investing the same amount monthly (instead of all at once) spreads out the risk of buying at a bad moment.

The easiest way to diversify

You do not need to manually buy 500 companies. Index funds and ETFs do this for you. A single S&P 500 fund holds slices of 500 large US companies. A "total world" fund holds thousands across every developed and emerging economy.

For most beginners, the right move is some flavor of "buy the world": one or two broad index funds, hold them for decades, contribute regularly. You will outperform most active investors not because you are smart, but because you avoided the unforced errors that come with picking individual stocks.

The limits of diversification

Diversification cannot eliminate all risk, only specific kinds. There are two flavors worth knowing about.

Idiosyncratic risk is the risk that a specific thing goes wrong with one investment. The CEO commits fraud, a single product fails, a particular country has a debt crisis. Diversification is fantastic at eliminating this. Spread across enough things, no single failure can sink you.

Systemic risk is the risk that the entire system goes down at once. A global financial crisis, a pandemic, a major war. Diversification cannot help you here, because everything correlates to one in a real systemic event. The only protection against systemic risk is time and the long-run tendency for economies to recover.

The classic mistake

The most common diversification failure: holding lots of your wealth in your employer's stock. People do it because of stock options, employee purchase plans, or just loyalty. The result is a portfolio where if your company goes through a rough year, you both lose your savings and your income at the same time.

Lehman Brothers employees in 2008 lost their jobs and most of their retirement savings in the same week, because most of their savings were in Lehman stock. The companies they worked at had been around for over 150 years. None of that mattered. Concentration is a luxury you can only afford after you have already won.