Diversification & Asset Allocation

Total investment risk is often split into two parts, and the distinction is the key to why diversification works.

• •UNSYSTEMATIC (specific / diversifiable) risk is unique to a single company, industry or issuer — a profit warning, a product recall, a fraud, a management change. It affects only that holding or a narrow group.
• •SYSTEMATIC (market / non-diversifiable) risk affects the market as a whole — interest-rate moves, recessions, inflation, geopolitical shocks. It cannot be removed by holding more securities of the same kind.

Diversification reduces UNSYSTEMATIC risk: as more, genuinely different holdings are added, company-specific ups and downs increasingly cancel out. It CANNOT remove systematic risk, which is the risk of being in the market at all.

Because unsystematic risk can be diversified away at little or no cost, an investor should not expect to be rewarded for bearing it; the return premium in efficient markets is for bearing systematic risk that remains after diversification.

A diversified portfolio holds many positions so that no single loss is large relative to the whole. Spreading money across different companies, sectors, geographies and asset classes means the events that hurt one holding are usually unrelated to those that hurt another.

As holdings are added, the volatility of the overall portfolio typically falls faster than its expected return — this is the 'only free lunch' in investing: a better risk-for-return trade-off without necessarily sacrificing expected return.

The benefit diminishes with quantity: the first additions cut risk sharply, but each extra holding removes less and less specific risk. Beyond a point the portfolio's risk approaches the market's systematic risk and further names add little.

Diversification must be genuine. Holding many stocks that all move together — for example many companies in the same industry or country — leaves the portfolio concentrated in disguise and removes little specific risk.