"Don't put all your eggs in one basket" is the folk version. The more useful version distinguishes between two kinds of risk and explains which one diversification can address.
Two Kinds of Risk
Modern portfolio theory, formalized by Harry Markowitz in a 1952 paper that later earned him a Nobel Prize, separates investment risk into two categories.
Idiosyncratic risk (also called specific or unsystematic risk) is the risk associated with a single company, industry, or asset. A particular firm losing a major customer, a CEO scandal, or a product recall are idiosyncratic events. They affect that company and its close peers, not the broader market.
Systematic risk (also called market risk) is the risk that affects the broader market or the economy as a whole. Recessions, interest rate changes, and global crises move many assets at once.
The fundamental insight of diversification is that holding many different assets dilutes idiosyncratic risk. If a portfolio contains one stock that loses 50% of its value due to a company-specific event, the impact on the total portfolio depends entirely on how large a position that stock represented. Spread across many holdings, the impact of any single bad outcome is smaller.
What Diversification Cannot Do
Diversification reduces idiosyncratic risk. It does not eliminate systematic risk. In a broad market downturn, the prices of many assets fall together. A well-diversified equity portfolio in 2008, or in early 2020, would still have suffered substantial losses, because the underlying event affected nearly every part of the market simultaneously.
This is sometimes summarized as: diversification protects against the failure of individual investments, not against the failure of the market as a whole.
How Many Holdings Is Enough?
Academic studies on this question, going back to research by Lawrence Fisher and James Lorie in the 1970s and many others since, generally find that the marginal reduction in idiosyncratic risk falls off sharply after roughly 20 to 30 well-chosen individual stocks. Beyond that point, additional positions reduce variance only modestly.
For investors using broad index funds or ETFs, the question is largely moot. A single fund tracking a major equity index can hold hundreds or thousands of stocks, providing extensive diversification within that asset class with one purchase.
Diversifying Across Asset Classes
Diversification within a single asset class — owning many stocks rather than one — addresses idiosyncratic risk in that class. Diversifying across asset classes — combining stocks, bonds, real estate, and possibly other holdings — addresses some, though not all, systematic risk by combining assets whose returns historically have not moved in lockstep.
The historical correlation between U.S. stocks and U.S. Treasury bonds, for example, has been low and at times negative over multi-decade periods. That has made stock-and-bond portfolios less volatile than either component alone in many environments. Correlations are not constant; in some crises, traditionally uncorrelated assets have moved together. The general point — that combining imperfectly correlated assets can reduce overall portfolio variance — remains structurally true.
Concentration Has Its Defenders
Not every investor diversifies broadly. Some experienced investors deliberately concentrate their holdings, on the theory that thorough analysis of a small number of companies can produce returns that justify the additional risk. Warren Buffett and others associated with that school have written extensively on the subject.
The argument and the counterargument both have respectable bodies of literature behind them. The empirical observation, drawn from decades of fund performance data, is that broad diversification has been a remarkably difficult baseline for concentrated active strategies to beat consistently after fees.
The Practical Read
Diversification is not a guarantee against loss. It is a structural way to ensure that no single bad outcome — a fraud, a bankruptcy, a sector collapse — destroys a disproportionate share of a portfolio. That is a meaningful, if limited, form of protection, and it is essentially free of charge once the right vehicles are in place.
This article is for general informational and educational purposes only and does not constitute financial, investment, tax, or legal advice. Consult a qualified professional before making any investment decision.