I have been a firm believer of MPT after reading a lot of Bernstein's books and podcasts, however the more I read into the theory elsewhere the more criticism I see from others. The main criticism surrounding MPT is that asset correlation is constantly changing - there is no point constructing a portfolio with back-tested data when the future correlation will certainly change. The efficient frontier is forever moving. There are some to even suggest that the correlation between stocks and bonds could change and become synchronized at some stage, rendering the old 60/40 portfolio useless in terms of risk reduction. There certainly have been long periods where bonds outperformed stocks, which you would have suffered if in an 100% equity allocation. What do you think of MPT? Is it still the gold standard of portfolio construction? Are the criticism simply from active investors trying to justify moving away?