Complex sentence: The theory of portfolio diversification was originated by Markowitz (and many others) and effective diversification requires management of the correlation between the asset returns and the liability returns, issues internal to the portfolio (individual holdings volatility), and cross-correlations between the returns.
Split into simpler sentences: The theory of portfolio diversification was originated by Markowitz (and many others). Effective diversification requires management of the correlation between the asset returns and the liability returns, issues internal to the portfolio (individual holdings volatility), and cross-correlations between the returns.