The Investment Scientist

Rebalancing “Bonus” in RSP Outperformance Over SPY

Posted on: January 15, 2014

images-58In the arena of academic finance, the debate over whether a rebalancing “bonus” exists or not has become somewhat of a religion!

Those who are ardent believers of an efficient market such as Nobel Prize winner Eugene Fama usually believe all returns should be the result of taking risk and that simple actions like rebalancing periodically should not produce additional returns.

Those who believe the market is emotion-driven, such as Nobel Prize winner Robert Shiller, believe in a rebalancing “bonus”. Since the market is either over-priced or under-priced from time to time, rebalancing allows us to take advantage of this market mispricing.

The return differential of RSP vs SPY provides an excellent control experiment to test whether this illustrious rebalancing “bonus” actually exists. SPY and RSP invest in the same 500 largest stocks of the US. SPY being a cap-weighted fund, does not require rebalancing, while RSP being a equally weighted fund requires periodic rebalancing.

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Last week I wrote a post entitled Solving the Puzzle of S&P 500 Equally Weighted Index Outperformance.” The conclusion is that most of the outperformance is due to the additional risk taking of RSP. In both regressions, the alphas of RSP and SPY are both NOT significantly different from zero. So it appears Eugene Fama wins hands down.

But wait a minute! RSP has a positive alpha while SPY has a negative one. To dig deeper, I ran a regression of the return differentials over the three risk factors.

R(RSP) – R(SPY) = alpha_diff + beta1_diff*(Rmkt – rf) + beta2_diff*SML + beta3_diff*HML

I got the following:


Standard Error

T Stat






















This basically confirms the conclusion of the previous post. That is, even though there is a huge return differential that is by and large accounted for by risk loadings (beta1, beta2 and beta3), the alpha differential is statistically insignificant.

However, it is not economically insignificant since the monthly alpha differential is 0.0341% and the annual differential is 0.41%. This is a material difference, consistent with other research which shows that a rebalancing “bonus” is usually in the magnitude of 40 to 50 basis points.

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Michael Zhuang is principal of MZ Capital, a fee-only independent advisory firm based in Washington, DC. He is also a regular contributor to Morningstar Advisor and Physicians Practice. To explore a long-term wealth advisory relationship, schedule a discovery meeting (phone call) with him.

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