Why diversification still matters, especially when one market is winning

Diversification is one of the oldest principles in investing. It is also one of the easiest to ignore when markets are performing well. When a particular asset class, market or index is consistently outperforming, it can feel rational to concentrate capital where returns appear strongest. Yet history repeatedly shows that this behaviour often increases risk at exactly the wrong time.

At its core, diversification is about managing uncertainty. No investor knows which asset class, region or sector will perform best in the future. By spreading investments across different assets that do not all move in the same direction at the same time, investors reduce the risk that a single event or downturn will materially damage their long‑term outcomes.

The behavioural trap of recent performance

One of the most common reasons investors resist diversification is recency bias. This is the tendency to assume that what has performed well recently will continue to do so indefinitely. Strong returns create confidence, confidence becomes conviction and conviction often leads to over‑concentration.

I was reminded of this recently through a conversation with a client I met socially. He had built a substantial portfolio invested almost entirely in one index fund. Over the last two years, this strategy had worked exceptionally well. The index had outperformed many diversified local and global portfolios, and he saw little reason to change what was clearly delivering results.

When diversification was raised, his response was simple. Why dilute returns by adding assets that were lagging? In his view, diversification was something investors needed when they were uncertain, not when the data seemed clear.

When the cycle turns

Markets, however, do not move in straight lines. What followed was a period of market stress and weaker equity performance. As the index fund he was invested in declined, his portfolio experienced a far sharper drawdown than portfolios that were more broadly diversified across asset classes and regions.

In contrast, well‑diversified portfolios held up more resiliently. Offshore exposure provided a hedge against local market weakness. Defensive assets such as bonds and cash reduced volatility. While diversified portfolios also experienced losses, the magnitude was materially lower, and recovery was faster.

The key lesson was not that the index fund he was invested in is a poor investment. It remains a core building block for many investors. The lesson was that concentration risk is often invisible during strong markets and painfully obvious during downturns.

What diversification actually achieves

Diversification is not about maximising returns in any single year. It is about improving the probability of achieving long‑term goals with fewer unpleasant surprises along the way.

A well‑diversified portfolio typically spreads capital across:

• Different asset classes such as equities, bonds, property and cash
• Multiple geographies, including South Africa and offshore markets
• Various sectors and investment styles
• More than one investment manager or strategy

These components do not all perform well at the same time. That is precisely the point. When one area underperforms, another may hold steady or outperform, smoothing overall returns and reducing the emotional pressure that leads investors to make poor decisions at the wrong time.

Diversification and investor behaviour

An often-overlooked benefit of diversification is our behaviour as investors. We are far more likely to stay invested when portfolio volatility is controlled. Large drawdowns increase the temptation to sell at market lows, locking in losses and undermining long‑term outcomes.

By reducing volatility and drawdown risk, diversification helps investors remain disciplined. This, in practice, can be just as important as asset selection or market timing.

A principle that remains relevant

The story of concentrated success followed by painful underperformance is not unique. It repeats itself across markets, asset classes and generations. Diversification may feel unnecessary when returns are strong, but that is often when it is most needed.

For investors with long‑term objectives, diversification is not a defensive compromise. It is a deliberate strategy to manage uncertainty, protect capital during downturns and improve the likelihood of reaching financial goals over time.

Markets will continue to rotate, cycles will turn, and yesterday’s winners will not always be tomorrow’s leaders. A diversified portfolio is not designed to predict those changes, but to be resilient when they inevitably occur.