Summer is often associated with taking a well-earned break, spending more time outdoors, or enjoying a holiday.

Investment markets have long had their own association with the season. The old market saying “sell in May and go away”, popularised during the 1950s, reflects the belief that equity markets tend to underperform during the summer months.

Whilst there is some historical evidence to support this pattern, it is far from a reliable investment strategy and remains widely debated.

What does the data suggest?

The “sell in May and go away” adage is based on the observation that equity markets have, on average, delivered lower returns between May and October than between November and April.

Since 1990, the S&P 500 has generated an average return of approximately 3.0% during the May to October period, compared with around 6.3% between November and April.

However, the evidence is far from conclusive. Looking at different market indices or extending the time period can produce very different results. For example, analysis of the S&P 500 dating back to the 1930s shows that, in several decades, the summer months actually outperformed the winter period.

Individual years also vary considerably. In both 2009 and 2020, summer returns significantly exceeded those achieved during the preceding winter months.

Why might summer markets behave differently?

There is no universally accepted explanation for why seasonal patterns appear in financial markets.

Lower trading volumes during the holiday season are often cited as one possible factor, as reduced market participation can influence liquidity and short-term price movements. Investor sentiment, institutional portfolio rebalancing, and seasonal changes in risk appetite have also been suggested as contributing factors.

However, these influences are often outweighed by more significant drivers of investment returns, including interest rates, inflation, corporate earnings, economic growth, geopolitical developments, and monetary policy.

In practice, markets rarely move because of the calendar alone.

The risk of trying to time the market

The key question is not whether summer returns have occasionally been weaker, but whether acting on this historical pattern improves long-term investment outcomes.

Moving in and out of markets requires two correct decisions: when to sell and when to reinvest. Getting either decision wrong can result in missing periods of strong market performance.

This is often easier said than done. Research has consistently shown that missing just the market’s 10 best trading days over a 20- to 30-year investment period can reduce long-term returns by more than half. Many of these strongest trading days also occur shortly after periods of market weakness, rather than following a seasonal pattern.

Frequent trading can also create transaction costs, potential tax consequences, and unnecessary disruption to a long-term investment strategy.

For many investors, remaining appropriately invested has historically proven to be a more reliable approach than attempting to predict short-term market movements or seasonal trends.

Investment returns are driven far more by asset allocation, diversification, corporate earnings, interest rates, inflation, valuations, and economic growth than by the month in which capital is invested.

Structuring portfolios for the long-term

Whilst seasonal market patterns can provide useful context, they form one small part of a much broader investment picture.

At Patterson Mills, we believe successful investing is built on disciplined portfolio construction, regular reviews, and ensuring that investment strategies continue to reflect changing objectives, risk tolerance, and personal circumstances over time. 

Rather than reacting to short-term market trends or seasonal patterns, we work with clients to ensure their portfolios remain appropriately diversified and aligned with their long-term financial goals. 

If you would like to review whether your current investments remain aligned with your circumstances, get in touch with us today and book your initial, no-cost and no-obligation meeting.

Send us an e-mail to contactus@pattersonmills.ch or call us direct at +41 21 801 36 84 and we shall be pleased to assist you.

Diversification is often cited as one of the most important principles in successful investing, and history has generally supported that view.

By spreading investments across different assets, sectors, regions, and economic drivers, diversification helps reduce concentration risk and avoids excessive reliance on any single investment outcome.

However, diversification is not designed to eliminate risk altogether.

During periods of significant market stress, investments that would normally behave differently can respond in a similar way due to the same underlying pressures.

This is known as correlation risk.

What is correlation risk?

Correlation risk is the danger that assets, strategies, or economic factors may become more closely linked than expected, reducing the benefits of diversification and increasing potential losses during market downturns.

When two investments are highly correlated, they tend to rise and fall at similar times. When correlation is low, their returns are more independent of one another.

The challenge for investors is that correlations are not fixed and can change significantly over time. Assets that appear well diversified during normal market conditions may become increasingly correlated during periods of economic uncertainty or market stress, reducing the protection that diversification would otherwise be expected to provide.

When markets move together

Periods of rising correlation do not mean diversification has become ineffective. Rather, they reflect the fact that many investments are ultimately influenced by a number of the same underlying economic and market forces.

During periods of heightened volatility, investors often reassess risk across large parts of the market simultaneously. This can result in broad selling across multiple asset classes, sectors, and regions, even where the underlying fundamentals remain different.

As a result, investments that would normally exhibit lower levels of correlation can begin moving more closely together, particularly over shorter periods.

Many growth-oriented investments ultimately share common drivers including:

• Economic growth 
• Corporate profitability 
• Interest rates 
• Investor confidence 
• Liquidity conditions

When these factors deteriorate, many growth assets can come under pressure at the same time.

However, this does not mean all investments are exposed to identical risks to the same degree. Even within equity markets, different regions, sectors, and companies are influenced by different economic conditions, valuations, competitive advantages, and long-term growth drivers.

Why diversification still matters 

Equities have historically been one of the primary drivers of long-term capital growth. Consequently, accepting some degree of correlation risk is often an unavoidable aspect of seeking higher long-term returns.

The aim is not to eliminate correlation risk altogether. Instead, it is to reduce unnecessary concentration by spreading exposure across a broad range of investments, regions, sectors, and economic drivers.

For example, a portfolio invested entirely in large US technology companies may deliver strong returns during favourable market conditions. However, it would also be heavily exposed to developments affecting a relatively narrow part of the market.

By contrast, a globally diversified growth portfolio spreads exposure across different countries, industries, companies, and sources of return. While many of the underlying investments may still be influenced by the broader forces affecting equity markets, the portfolio is not solely dependent on a single sector, region, or economic outcome.

In practice, diversification often involves combining investments across a range of asset classes and markets, including:

• Developed market equities 
• Emerging market equities 
• Government bonds 
• Corporate bonds 
• Cash 
• Property and infrastructure 
• Other real assets 

These assets may not always move independently, particularly during periods of market stress. However, they can respond differently over longer periods of time and under different economic conditions, which is why diversification remains a key part of portfolio construction.

Structuring portfolios for long-term resilience

Correlation risk reminds investors that markets are constantly evolving. Relationships between investments that appear stable during one period may look very different when economic conditions, investor sentiment, interest rates, or geopolitical events change.

For this reason, portfolio construction should not rely solely on how investments have behaved in the past. Consideration must also be given to the underlying drivers of returns and how different investments may respond under a range of future market conditions.

At Patterson Mills, we work with clients to build portfolios that seek to balance long-term growth objectives with effective risk management. While no portfolio can eliminate market risk entirely, careful diversification, regular review, and disciplined portfolio construction can help investors remain resilient in an environment where market relationships are rarely static.

If you would like to review whether your current investments remain appropriately diversified and aligned with your long-term objectives, get in touch with us today and book your initial, no-cost and no-obligation meeting.

Send us an e-mail to contactus@pattersonmills.ch or call us direct at +41 21 801 36 84 and we shall be pleased to assist you.