Diversification is the investment strategy of spreading capital across different assets, sectors, geographies, or asset classes to reduce the impact of any single investment's poor performance on the overall portfolio. The core principle: combining assets with low or negative correlations lowers portfolio risk without proportionally reducing expected return.
Modern Portfolio Theory (MPT), developed by Harry Markowitz in 1952 and recognized with the Nobel Prize in Economics, formalized diversification: an investor can reduce portfolio variance (risk) by combining assets whose returns do not move in lockstep. Correlation is the key metric — assets with a correlation near -1 perfectly offset each other; assets near +1 move together. Practical diversification occurs at multiple levels: (1) across individual securities within an asset class (not concentrating in one stock); (2) across asset classes (stocks, bonds, real estate, commodities); (3) across geographies (domestic vs. international); (4) across time horizons (dollar-cost averaging spreads market entry timing). Index funds and ETFs are the most accessible diversification tools for individual investors. A single broad market index fund can hold thousands of securities. The SEC's investor.gov and FINRA both cite low-cost, broadly diversified index funds as the baseline recommendation for most long-term retail investors. Diversification does not eliminate market risk (systematic risk) — it reduces company-specific or sector-specific risk (unsystematic risk).
Yes, in theory. Beyond a certain number of holdings (roughly 20–30 stocks per research from MPT), incremental diversification benefit diminishes. Over-diversification in actively managed portfolios can dilute high-conviction ideas. Index funds avoid this by design — they hold the market.
No. Diversification reduces unsystematic (company/sector-specific) risk but cannot eliminate systematic (market-wide) risk. In a broad market downturn, a diversified portfolio will still decline, just less than a concentrated one.