How to quantify the influence of correlations on investment   diversification
  When assets are correlated, benefits of investment diversification are reduced. To measure the influence of correlations on investment performance, a new quantity - the effective portfolio size - is proposed and investigated in both artificial and real situations. We show that in most cases, the effective portfolio size is much smaller than the actual number of assets in the portfolio and that it lowers even further during financial crises. 
Matus Medo, Chi Ho Yeung, Yi-Cheng Zhang