Diversification: The Only Free Lunch in Investing

Economist Harry Markowitz won a Nobel Prize for, in part, formalizing a simple idea your grandmother already knew: do not put all your eggs in one basket. In finance, diversification is often called the only free lunch — a way to reduce risk without giving up expected return.

The mechanism

Different assets do not move in lockstep. When one zigs, another may zag. By holding many investments whose ups and downs do not perfectly align, the bumps partially cancel out, and your overall portfolio rides smoother than its individual parts.

Two kinds of risk

• Specific risk is tied to one company: a bad CEO, a product recall, an accounting scandal. This risk is uncompensated — the market does not pay you to take it, because you could have diversified it away.
• Market risk affects everything at once: recessions, rate shocks, pandemics. This risk cannot be diversified away, and it is the risk you are actually paid to bear.

Diversification's job is to eliminate the first kind so you are left holding only the risk that comes with a reward.

How to diversify in practice

• Across companies: dozens or hundreds, not three or four. A broad index fund does this in one purchase.
• Across asset classes: stocks, bonds, real estate, cash.
• Across geographies: not just your home country.

What it is not

Diversification will not stop your portfolio from falling in a broad market crash — everything can drop together for a while. It also will not maximize your return; concentration does that, in the rare cases it works. Diversification is insurance against being catastrophically wrong about any single bet.

The takeaway

You are not paid for risks you could have avoided. Spread your money across many holdings and several asset classes, and you keep the reward of investing while shedding the risks that offer no reward in return.