How Diversification Helps Reduce Portfolio Risk

The first point to understand is this: diversification is a risk management strategy, not a strategy to increase returns. The aim is not to maximize gains, but to reduce the fluctuations and downside risk of a portfolio over time. When investors talk about diversification, they usually mean spreading investments across different dimensions, for example:

• Asset classes: holding both equities and bonds, rather than equities only
• Sectors and industries: not concentrating exclusively on technology companies
• Geographic regions: investing in domestic and international equities
• Different companies: owning shares in several companies, rather than only one

Each of these levels offers a different form of protection. For example, a portfolio that is diversified only within one sector, such as Swiss bank stocks, is not truly diversified, even if it includes twenty different companies.

The risk-reducing effect of diversification is linked to the concept of correlation — a statistical measure of how the returns of two investments move in relation to each other over time.

• Investments with high positive correlation tend to move in the same direction at the same time. Owning ten highly correlated stocks offers little protection if all of them fall at once.
• Investments with low or negative correlation tend to move more independently or in opposite directions. If one investment falls, another may remain stable or rise.

This is why combining different asset classes — for example, equities with bonds or real estate investment instruments — is often more effective than diversifying within pure equity portfolios. The mix matters more than the number of holdings.

Modern Portfolio Theory, developed in the 1950s by Harry Markowitz, shows that for a given return target, risk can be reduced by combining investments that do not move in lockstep.

One point that many beginners underestimate: diversification cannot eliminate all risks.

Investment risk comes in two broad categories:

• Unsystematic risk (also called specific or idiosyncratic risk) refers to risks related to individual companies, sectors, or investments. This risk can be significantly reduced through diversification: if one company is affected by an accounting scandal or a particular sector faces stronger regulation, a broadly diversified portfolio is less exposed to that single event. This risk can be reduced by holding a larger number of equities.
• Systematic risk (market risk) affects the entire market or economy, for example recessions, interest rate changes, geopolitical crises, or global health events. Diversification offers little protection against systematic risk because in such phases, many investments may lose value at the same time, despite broad diversification.

Understanding this distinction is essential. Investors who expect diversification to act as a complete safety net may be caught off guard during broad market downturns.

Not necessarily. Fifty stocks from the same country and the same sector are less diversified than ten stocks spread across different industries, asset classes, and regions.

True diversification aims to reduce concentration risks — avoiding excessive exposure to a single source of risk, such as one company, one sector, one country, or one currency. A person whose entire portfolio consists of large-cap Swiss equities carries significant concentration risk, even if that portfolio includes all SMI stocks.

Adding other asset classes, such as ETFs tracking international indices or fixed income securities, can significantly improve diversification and optimize the portfolio’s risk-return profile.

These two terms are closely related but not interchangeable.

Asset allocation refers to the strategic allocation of a portfolio across the main investment categories — typically equities, fixed income investments, and cash. It depends on an investor’s risk capacity, risk tolerance, investment horizon, and financial goals.

Diversification is the practice of spreading investments within and across those asset categories to reduce concentration risk.

Simply put: asset allocation defines the basic structure of the portfolio; diversification fills that structure with content and provides the spread. A carefully constructed portfolio needs both.

It is possible to diversify “too much". Over-diversification, sometimes referred to in English as “diworsification,” describes a portfolio with so many positions that the additional benefit of each further investment becomes negligible.

At a certain point, adding more positions does not meaningfully reduce risk further; it only increases complexity, may raise transaction costs, and can make it harder to track and rebalance the portfolio effectively.

Most studies suggest that a portfolio of around fifteen to thirty carefully selected, genuinely low-correlated investments captures most diversification benefits. Beyond that, the additional improvements in risk reduction are limited.

Even experienced investors make mistakes when applying diversification. The most common include:

• Confusing quantity with quality: Holding many funds or ETFs that track the same or very similar indices does not create true diversification, but overlap and redundancy.
• Ignoring correlation: Adding investments that differ only in name but behave similarly in practice offers little additional protection. In periods of market stress, correlations between asset classes can rise significantly.
• Neglecting geographic diversification: Many investors have a pronounced home bias. That means they overweight their domestic market more than would make sense in a global context. For Swiss investors, this can mean holding mainly Swiss or European equities while underweighting other regions.
• Failing to rebalance regularly: Over time, market movements change the weighting of individual positions. A portfolio that was originally balanced can shift significantly if one asset class performs above average for an extended period.
• Skipping rebalancing: Periodically adjusting positions back to the intended structure is a necessary maintenance step that many investors overlook.
• Treating diversification as a substitute for analysis: Diversification reduces the cost of being wrong about a single investment. But it does not replace understanding what you hold in your portfolio and why.

For private investors who are looking at diversification for the first time, the following principles are particularly useful:

• First, understand your risk tolerance and investment horizon. These two factors should determine your asset allocation before you decide how to diversify within each category. A younger person with a long investment horizon can generally tolerate more short-term fluctuations and allocate more heavily to equities; a person close to retirement is more likely to prioritize capital preservation and focus more strongly on fixed income investments.
• Use broadly diversified ETFs or investment funds as a cost-effective starting point. A single ETF that tracks a global index gives you immediate access to hundreds or thousands of companies from different countries and sectors — a pragmatic starting point for diversification.
• Review your portfolio regularly. Markets change, and so does your personal situation. Rebalancing at fixed intervals helps align your portfolio with its original target structure.