Unsystematic risk
Unsystematic risk is the risk attached to one company or industry, such as a recall or a lost lawsuit, and holding many unrelated positions removes most of it.
Unsystematic risk is everything that can go right or wrong at one company, or in one industry, without the rest of the market noticing. It is also called specific, idiosyncratic or diversifiable risk, and the third name is the useful one. It is the part that spreading money across unrelated positions genuinely takes away.
- A failed trial, a product recall, or a warehouse that burns down.
- A fraud, a resignation, a lost lawsuit, or a large contract that does not renew.
- A rule change that lands on one industry and leaves the rest alone.
Because it can be removed at almost no cost, asset pricing theory holds that nobody is paid to carry it. Expected return is compensation for systematic risk only, so concentrating in a single stock adds uncertainty without adding anything to the return the theory says you should expect in exchange. That is the entire argument for holding many positions rather than a favourite few.
The mistake is confusing a long list of tickers with diversification. Twenty companies in one industry share the same specific risks, so their bad news tends to arrive together and the count does little. The same trap catches anyone whose salary, share options and pension all depend on one employer, which is one bet placed three times. What matters is the correlation between holdings, not how many of them there are.