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These two charts illustrate the importance of your investment horizon in evaluating diversification opportunities. Correlations are usually computed using monthly returns data, because that's what is readily available. But if your investment horizon is longer than one month, correlations based on monthly data can be very misleading: you should be using returns over longer horizons to evaluate diversification for your portfolio.
The first chart shows the correlation between sector returns and returns in the broad stock market using investment horizons increasing from one month to 60 months (five years). Most sectors show what I call an "upward-sloping term structure of correlations," meaning that correlations over longer investment horizons are higher than correlations based on one-month returns. For example, one-month returns suggest that Telecom stocks have provided good diversification against the rest of the stock market, with a correlation of just 0.60 since the beginning of 1990--but over five-year periods the correlation between the Telecom sector and the broad market has actually been 0.93, meaning that Telecom stocks have provided very little diversification for retirement investors, endowments, foundations, and other investors with longer horizons. In essence, while events specific to a given sector matter in the short run, in the long run you're just getting exposure to the business (stock market) cycle and it doesn't matter whether you get that exposure through Telecom stocks, Financial stocks, Industrial stocks, or others.
The only exceptions are the Energy sector (shown) and the Basic Materials sector (not shown), which have had a "downward-sloping term structure of correlations" with lower correlations over longer horizons. That's because Energy and Basic Materials stocks generally give exposure to a separate asset class, the Commodities asset class, even though the exposure comes through the stock market.
The other major exception is shown on the second slide: listed equity REITs have had a very strongly downward-sloping term structure of correlations. In fact, the REIT-stock correlation over five-year horizons is only in the 0.20-0.30 range, meaning that investors with long horizons have benefited tremendously from the power of diversification. That's because REITs, too, give exposure to a very different asset class--the Real Estate asset class--even though the exposure comes through the stock market.