Portfolio Diversification Check

Modern Portfolio Theory shows that investment risk can be reduced by combining assets with low or negative correlation. Statically, the variance of a portfolio is given by σ_p² = Σ w_i² σ_i² + Σ Σ w_i w_j σ_ij, where σ_ij is the covariance between assets. Diversification eliminates idiosyncratic risk, leaving only systemic market risk, allowing investors to maximize return for a given level of risk.

Diversification is not about the number of holdings but about how differently they behave. If ten funds all track large US tech, they rise and fall together — a correlation near +1 — so you effectively hold one bet ten times. This tool measures those pairwise relationships so you can spot hidden overlap: a lower average correlation, not a longer list of tickers, is what actually smooths your returns.

A calm-market snapshot can badly overstate your protection. Correlations between risky assets are not fixed — they tend to jump toward +1 during panics, exactly when you most need them apart. In the 2008 crisis and the March 2020 crash, assets that normally drifted independently fell together. So a single lookback window gives one average number that can hide this regime shift, and diversification often fails at the worst possible moment.