Psychologie a chování
Investment Mistake of the Month: Why Putting Everything in One Sector Leads to a Quiet Disaster
Key takeaways
- Sector concentration exposes a portfolio to a single specific risk: regulation, cyclicality, or technological disruption of that industry.
- Diversification across sectors does not mean lower returns — historically it delivers better return per unit of risk.
- Psychological trap: a sector that is growing attracts more money, so investors add more just before the peak.
- The solution is a globally diversified ETF or an intentional allocation of the portfolio across different sectors.
- Home bias (overweighting Czech or European stocks) is a special case of sector concentration.
The mistake looks like this: you have strong results in technology (or energy, or real estate), you add more from the same sector, then a bit more — and suddenly 80% of the portfolio is in a single industry. It feels like expertise. It is not.
Why It Happens
The brain likes to extrapolate. A sector that has grown for three years "will keep growing" — that is the cognitive shortcut we call recency bias. It combines with another effect: when you know more about a topic than others (or think you do), you feel safe adding more. The result is sector overweighting that the investor perceives as competence but the market reads as concentrated risk.
What Can Actually Go Wrong
- Regulatory shock: an entire sector can overnight face new regulation (tax, ban, obligations) — tech, pharma, and energy have all experienced this.
- Technological disruption: a new technology can devalue an entire sector (see the impact of EVs on traditional automakers).
- Cyclical correction: sectors such as energy or real estate are strongly cyclical — and the cycle turns without warning.
- Crisis correlation: in a panic sell-off, all stocks in a sector fall simultaneously because investors are selling the "theme."
How to Fix It
Step one: find out which sectors you hold and how much. Step two: if one sector exceeds 30%, consider rebalancing. Step three: a core position in a globally diversified ETF (such as an All-World fund) will almost automatically prevent extreme sector concentration. More on diversification in how to build your first portfolio or in All-World vs. S&P 500. It is also worth noting that risk wears many faces — and sector concentration is one of the most treacherous.
FAQ
Is sector concentration really that dangerous?
Yes — specifically because it develops gradually and unconsciously. An investor keeps adding what is rising and only later discovers that 70% of the portfolio is in a single industry. In a correction, the pain is extreme.
What proportion of one sector is acceptable?
The general rule says a maximum of 25–30% per sector. A global index ETF naturally polices this — technology typically makes up about 20–25% of the index, and the rest is automatically diversified.
Does this apply to Czech home bias too?
Yes, home bias — overweighting domestic or regional stocks — is the same problem. The Czech stock exchange is small and highly concentrated. A portfolio composed of Czech stocks is sectorally and geographically undiversified.