"Don't put all your eggs in one basket" is one of the oldest pieces of investment advice — and one of the most mathematically sound. Diversification works because assets don't all move together at the same time in the same direction. When one investment falls, others may hold steady or rise, cushioning the blow.
What diversification actually does
Risk in a portfolio comes in two flavours:
- Specific (idiosyncratic) risk: The risk that a particular company fails — management fraud, a product recall, a specific industry disruption. This can be largely eliminated through diversification.
- Market (systematic) risk: The risk that the whole market falls — recessions, panics, global crises. This cannot be diversified away; if everything falls together, no amount of diversification helps.
Beyond just owning more stocks
True diversification isn't just holding 100 different companies — if they're all UK banks, they'll all fall together in a financial crisis. Meaningful diversification requires spreading across:
- Asset classes: Stocks, bonds, real estate, commodities move differently
- Geographies: US, Europe, Asia, emerging markets have different economic cycles
- Sectors: Tech, healthcare, energy, consumer staples respond differently to the same events
- Company sizes: Large-cap, mid-cap, small-cap have different risk/return profiles
Diversification is the only free lunch in investing. — Harry Markowitz, Nobel Prize winner in Economics
The correlation trap
Assets that seem uncorrelated in normal times can become correlated in a crisis — when panic strikes, investors sell everything. In 2008, both stocks and many supposedly "safe" assets fell together. True diversification requires assets that hold up in extreme conditions — typically government bonds, gold, and cash — not just ones that look different in a spreadsheet.
What this means for you
A single global index fund (MSCI World or S&P 500) already gives you exposure to thousands of companies across dozens of countries. That's substantial diversification for a very low cost. Adding bonds provides further resilience. The point where further diversification adds diminishing returns comes sooner than most people think — 20–30 carefully chosen stocks eliminates most specific risk; 2–3 asset classes handles most of the rest.