What Happened to the Index Effect? A Look at Three Decades of S&P 500 Adds and Drops – Research


  • Minimum volatility is part of a broader group of defensive strategies that have been in existence for decades. They are based on the low volatility anomaly, the phenomenon that lower-risk stocks outperform over time, contradicting the conventional wisdom that risk and reward go hand in hand.
  • Low volatility strategy indices attempt to exploit this anomaly systematically. The typical behavior patterns of low volatility strategies are that they go up less when the market is up and go down less when the market is down. They offer protection in down markets and participation in up markets.
  • More than with most factor strategies, the potential value added of low volatility strategies is largely dependent on market dynamics. Dispersion of returns tends to be higher in times of crisis; this disparity gives defensive strategies such as low volatility a leg up.

research profiling minimum volatility exhibit 1


Following a few years of significant market gains, enthusiasm for low volatility strategies has waned, particularly compared with the period after the 2008 Global Financial Crisis. This is understandable since protection is probably not top of mind when things are going well and are seemingly on an upward trajectory.


Low volatility strategies explicitly aim to deliver a pattern of returns relative to the market. Their goal is to reduce risk (volatility), and that goal is constant in both good times and bad.

Low volatility is a characteristic. Low volatility accompanied by outperformance is an anomaly. The phenomenon of lower-risk assets also outperforming higher-risk assets over time was noted by academics almost half a century ago.  Flouting the conventional wisdom that risk and return
go hand in hand, this phenomenon was dubbed the low volatility anomaly. Outperformance does not occur at all times (particularly in strong market performance cycles), but the anomaly has been observed universally across different markets and asset classes.

When it comes to low volatility portfolios, there are different approaches to index construction that yield different characteristics and results. In the U.S., the S&P 500 Minimum Volatility Index is one way to pursue lower risk in a systematic way.

The methodology underlying the S&P 500 Minimum Volatility Index relies on optimization, minimizing volatility subject to stock- and sector-level exposure constraints. Compared with a rankings-based methodology such as the one used for the S&P 500 Low Volatility Index, the optimized approach has typically resulted in less performance divergence from the benchmark.

research profiling minimum volatility exhibit 2

In the period from January 1991 through May 2021, the minimum volatility index delivered nearly the same return as the benchmark S&P 500, but at substantially lower risk—a 16% reduction. On a 10-year rolling basis, the S&P 500 Minimum Volatility Index’s volatility was consistently lower than the S&P 500 throughout the entire period (see Exhibit 3).

research profiling minimum volatility exhibit 3

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