If you’re making an argument for passive investing, you’d probably point out that it’s less expensive, it’s tax efficient, and has paid off over long periods of time. In a recent Bloomberg article, columnist Nir Kaissar describes the S&P 500 (the first index fund) as the “poster child” for passive investing, having returned 11.6% annually over the last 5 years (from June 2011 to May 2016) versus the 10.5% return of the S&P 500 Value Index fund (an actively managed fund).

Through May of this year, however, the S&P 500 Value has outperformed its passively-managed cousin with returns of 5.3% (versus 3.6%). Kaissar explains that, while investors were happy to accept the S&P 500’s bounty without question, they will undoubtedly want answers concerning its recent underperformance.

So how is this sausage made? Kaissar explains that buying into the S&P 500 is “not exactly the same thing as buying the market.” Rather, given that the fund is market capitalization-weighted, it is primarily an investment in only one hundred of the roughly 3,700 firms listed in the U.S. stock market. And,  Kaissar notes, these top one hundred stocks are by far the most expensive in the index with average price-to-earnings ratios of 34 (based on one-year trailing earnings of the most recent fiscal year). A few years ago, he says, it was the opposite.

The reality, Kaissar argues, is that the S&P 500 is a “bet on high-valuation stocks” which makes it no different than any other bet, whether on value or any other active strategy.