5.2. Concentrating Everything in One Company
Lesson objective
Understand position-size risk.
Even excellent research can be wrong or disrupted by an unpredictable event. An excessively large position turns a company-specific error into a personal financial problem.
Concentration also increases emotional pressure and may lead to selling at the worst possible time.
Appropriate position size depends on the portfolio, goals, and risk capacity. There is no universal percentage.
Concentration turns one mistake into a portfolio problem
The impact of a position depends on its weight, not only on how much the stock moves.
Approximate portfolio impact = Position weight × Position return
The arithmetic of concentration
If one company represents 60% of a portfolio and its stock falls 50%, that position alone subtracts about 30 percentage points from the portfolio: 60% × −50% = −30%. A 10% position experiencing the same fall would subtract about 5 points.
Diversification protects against being wrong in one specific way
It cannot prevent the whole market from falling, but it can reduce damage from company-specific events such as fraud, product failure, litigation, disruption, or an unexpected capital need.
| Concentration source | Hidden exposure | |---|---| | One company | Company-specific execution risk | | One industry | Common demand/regulatory cycle | | One geography | Political/currency/economic risk | | Similar business models | Same competitive shock | | Highly correlated assets | Positions may fall together |
Diversification is not about owning the maximum number of tickers. It is about avoiding a portfolio where many positions depend on the same economic outcome.
What you should remember
- Diversification protects against specific errors, not against every market decline.
Practice
Calculate the effect on a portfolio if one holding falls 50% when it represents 5%, 20%, and 50% of the portfolio.