What Is Diversification? How Spreading Holdings Changes Portfolio Risk

Diversification means spreading money across holdings that do not all move together, so that a loss in any one of them carries less weight in the whole result.

It is one of the first ideas in portfolio construction, and often reduced to a slogan. This page covers why diversification lowers the swings of a portfolio, why correlation matters more than the number of holdings, where its limit sits, how concentration is measured, and where Kresmion shows these figures. It is descriptive throughout.

The idea in one paragraph

FINRA's investor guide sums it up with the old adage about not putting all your eggs in one basket, and describes the benefit in terms of assets that are uncorrelated, meaning they react to economic events in ways independent of each other. The opposite condition has a name: concentration risk, which FINRA defines as the risk of amplified losses from holding a large portion of a portfolio in one investment, asset class or market segment. Diversification does not change what any single holding does. It changes how much each holding's result can move the total.

Why correlation is the lever

The return of a portfolio is the weighted average of the returns of its holdings. Its volatility is not. Volatility is measured as the standard deviation of returns, and when two holdings do not move in lockstep, part of their swings offset each other, so the portfolio's volatility comes out lower than the weighted average of the two.

How much lower depends on the correlation between them, a number from minus 1 (they always move in opposite directions) to plus 1 (they always move together). Take two holdings with the same 20 percent annual volatility, held half and half:

At a correlation of 1 there is no benefit at all: the two holdings are, for risk purposes, one holding. Every step down in correlation removes more of the swing. The formula behind the table is the standard two-asset one: portfolio variance equals the sum of each weight squared times its own variance, plus twice the product of the two weights, the two volatilities and the correlation. This is the insight the Nobel committee credited to Harry Markowitz in 1990: a portfolio's risk depends not only on each asset's own variance but on the covariances between every pair of assets.

How many holdings is enough, and where the limit is

Adding holdings helps, but each extra holding helps less. Suppose every holding has 20 percent volatility and the portfolio is split equally among them.

If the holdings were truly independent, volatility would keep shrinking toward zero. Real securities are not independent: most stocks respond to the same economy, the same interest rates and the same shifts in sentiment. With an average correlation of 0.3, volatility levels off toward about 11 percent however many names are added (20 percent times the square root of 0.3, or 10.95 percent), because that part of the risk is shared by all of them. The 1990 Nobel press release makes the same point: because returns on different assets are correlated in practice, risk cannot be totally eliminated however many securities a portfolio holds.

That splits risk into two parts. The part specific to one company (a failed product, a fraud, a lost contract) is what spreading across many holdings dilutes. The part common to the whole market remains, and a holding's sensitivity to it is what beta measures.

Counting holdings is not the same as diversifying

Ten technology stocks are ten holdings and, by the arithmetic above, much less than ten independent bets. FINRA lists this as concentration through correlated assets: investments in the same industry, region or security type tend to be highly correlated, so what happens to one is likely to happen to the others. Its examples include several municipal bonds all issued in the same state or region. The same applies to funds: two broad index funds that track overlapping indices hold many of the same companies (see what an index fund is and what an ETF is).

Measuring concentration

Two simple numbers describe how evenly a portfolio is spread, before any correlation is considered.

• Top-five weight. The share of the portfolio in its five largest holdings.
• Herfindahl-Hirschman Index (HHI). The sum of every holding's weight squared. A single holding scores 1. Four equal holdings score 4 x 0.25 squared = 0.25, and ten equal holdings score 0.10. One divided by the HHI gives an "effective number" of equal-sized holdings.

A worked example: weights of 40, 30, 20 and 10 percent give an HHI of 0.16 + 0.09 + 0.04 + 0.01 = 0.30, or about 3.3 effective holdings. Four names, but spread like three and a third.

Where Kresmion shows these figures

The Risk section of Kresmion's portfolio analytics shows top-five concentration and the HHI for the holdings you enter, with a band label next to the HHI: "concentrated" at 0.25 or above, "moderate" from 0.10, "diversified" below 0.10. Those cut-offs are Kresmion's own display labels, not a regulatory standard. With at least 20 daily returns the same section adds annualised volatility, beta and correlation against a benchmark you pick (SPY, QQQ, ACWI, BTC or a 60/40 mix of SPY and AGG). A separate Exposure section splits holdings by asset class, sector, country and currency. The portfolio needs a Kresmion account, is stored encrypted, and its analytics are computed in your browser.

For the correlation side, the free correlation regime tool tracks a 45-symbol universe across seven asset classes, comparing each pair's 30-day rolling correlation of daily returns with its own trailing 252-trading-day average and flagging pairs that have moved far from it. It shows how far a pair's recent correlation has drifted from its own baseline; it does not say what that pair will do next.

Honest limitations

Correlation is measured over a past window, and the figure changes: a pair that moved independently for a year can move together the next month, which is the drift the correlation regime tool exists to flag. The tables on this page are hypothetical holdings with round-number volatilities and constant correlations, chosen to show the arithmetic, not estimates for any real assets. Volatility measured over a calm window can be low while a large single exposure that rarely moves still carries the risk of a sudden jump. And diversification reduces the risk that one holding dominates; it does not remove the risk common to all of them, and it does not guarantee against loss.

Key takeaways

Frequently asked questions

Does diversification guarantee against losses?

No. It reduces the chance that one holding dominates the result, but the part of risk shared by the whole market remains. A diversified portfolio can still fall, and in a broad decline most of its holdings can fall together.

How many stocks does it take to be diversified?

There is no single number. In this page's table, with every pair at a correlation of 0.3, ten equal-weight holdings already remove most of the reduction available, and beyond about 30 the shared market risk is nearly all that is left. The answer depends on how correlated the holdings are, which is why ten stocks from one sector are less diversified than ten from different ones.

Does diversification lower returns?

The portfolio's return is the weighted average of its holdings' returns, so it lands between the best and the worst holding rather than matching either one. What diversification changes is volatility, which can fall below the weighted average of the holdings' volatilities when they are not perfectly correlated.

What is the Herfindahl-Hirschman Index in a portfolio?

It is the sum of each holding's weight squared, a measure of concentration borrowed from the study of market competition. It equals 1 for a single holding and falls as weight spreads out; one divided by it gives the effective number of equal-sized holdings.

Is an index fund diversified?

A broad index fund spreads money across many companies, which dilutes company-specific risk. It still carries the market's shared risk, and it is only as diversified as its index: a sector index fund concentrates in that sector.

This page is information, not investment advice.

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Source: FINRA, Asset Allocation and Diversification, https://www.finra.org/investors/investing/investing-basics/asset-allocation-diversification ; FINRA, Concentrate on Concentration Risk (15 June 2022), https://www.finra.org/investors/insights/concentration-risk ; The Royal Swedish Academy of Sciences, press release for the 1990 Prize in Economic Sciences (Markowitz, Miller, Sharpe), https://www.nobelprize.org/prizes/economic-sciences/1990/press-release/ ; portfolio volatilities and HHI figures computed by Kresmion for hypothetical holdings ; Kresmion correlation regime tool and portfolio analytics.

Kresmion Research.