Diversification is the investment principle: don't put all your eggs in one basket. Instead of betting everything on one stock, sector or currency, you spread capital across many independent sources of return. When one part of the portfolio loses, others can balance it out.

In practice this means owning stocks of different companies, bonds, and often geographic dispersion – investing in the US, Europe, emerging markets. Diversification does not protect against every loss, but it significantly reduces the risk of a catastrophic drop.

A common myth: 'I spread my money, so I will earn less'. In reality what matters is the risk-adjusted return. Diversification eliminates idiosyncratic risk (e.g. one company failing) without taking away the market risk premium.

Adding bonds or international stocks to a US stock portfolio historically did not lower the average annual return, but significantly reduced maximum drawdowns. Lower volatility allows the investor to weather crises more calmly and avoid emotion-driven decisions.

In the years 2000–2010 a portfolio based solely on US stocks recorded a lost decade. Meanwhile investors diversified geographically (emerging markets, Europe) achieved positive returns. This shows that spreading assets not only protects but can also improve long-term outcomes.