Psychologie a chování
Betting It All on One Card: Why Excessive Concentration Destroys Portfolios
Key takeaways
- Excessive concentration in a single stock or market is one of the most costly investment mistakes.
- Home bias is a special form of concentration — investors overweight stocks from their own country.
- Diversification reduces risk without having to sacrifice long-term return.
- A single global ETF largely solves the concentration problem.
- If you favor one stock, ask yourself: would I be willing to put my entire portfolio into it?
Excessive portfolio concentration — whether in one stock, one sector, or one market — is one of the most common and costly mistakes Czech investors make.
Why concentration is so tempting
Betting on a single card is psychologically attractive. One clear story, one company, one idea — it's simple, comprehensible, and exciting. Moreover, if the bet pays off, the returns are dazzling. The problem is that extraordinary returns carry extraordinary risk: for every stock that rises a thousandfold over a decade, there are dozens that go bankrupt.
Home bias as a special form of concentration
A special case of excessive concentration is home bias — the tendency of investors to assign disproportionately large weight to stocks from their own country. The Czech exchange makes up less than 0.05% of global market capitalization. Yet many Czech investors hold a large part of their portfolio in domestic names or the Central European region.
The result? The portfolio is strongly correlated with the Czech economy — and with risks you already carry through other means (employment, property, pension). This problem is discussed in detail in the article on global vs. domestic stocks.
Diversification as the cure
Diversification is not about mediocrity — it is about not betting the existence of the portfolio on a single outcome. The math is clear: a diversified portfolio reduces volatility without an equivalent loss of return, because not all assets decline simultaneously.
- One global ETF covers thousands of companies across dozens of countries.
- A combination of stocks and bonds reduces maximum drawdown.
- Regular investing via cost averaging reduces the risk of poor timing.
How to avoid the mistake in practice
Set a rule: no single position should make up more than 5–10% of the portfolio. For the vast majority of investors, the easiest route is one or two broad ETFs — how to put together such a portfolio is described in the first portfolio guide. What risk is and how to measure it is explained in the article on risk.
FAQ
Why is excessive concentration dangerous?
Because one company or one market can collapse, taking the entire portfolio with it. Diversification reduces this risk: not all assets fall at once, so losses in one part are offset by gains elsewhere.
What is home bias and why does it hurt?
The tendency to give disproportionate weight to stocks from one's own country. The Czech exchange is small and heavily concentrated in industry — excessive domestic exposure adds unnecessary risk on top of what you already carry through employment and property.
How can you quickly fix excessive concentration in a portfolio?
Stop adding to the over-concentrated position and direct new contributions to a broad ETF. When selling, keep taxes in mind — if the position is older than 3 years, the time-based tax exemption may apply.