The Global Market Index (GMI) is expanding its asset class horizons. Starting with the numbers as of December 31, 2012, GMI’s allocations will add foreign REITs and foreign high-yield bonds to the mix. Next week, when I publish the monthly update on asset class returns through January, these two betas will make their formal debut. (For new readers who are wondering what I’m talking about, here’s the latest monthly update of the major asset classes and GMI.) The reasoning behind this change is that the arrival of ETFs that track these markets makes it easy and cost-efficient to allocate to non-US REITs and high-yield bonds. Considering the history of these markets in context with other asset classes, there’s also a strategic/tactical argument for adding foreign REITs and foreign high-yield bonds to the investment opportunity set. The correlations between the new additions and the usual suspects is still fairly high, but it’s well short of perfect positive correlation. Accordingly, there’s opportunity in these betas for enhancing the rebalancing bonus, if only on the margins.