Investors waiting for U.S. stock indexes to test their August low should be aware that more than a third of equities are already there.

While the Standard & Poor’s 500 Index is 2.8 percent above its worst closing level of 2015, almost 35 percent of the gauge’s members have slipped back below their Aug. 25 price. The heavyweights are doing all the lifting: Apple Inc., Microsoft Corp. and Exxon Mobil Corp., the three largest companies by market cap, account for nearly one-fifth of gains since the market bottomed after a four-day selloff of 10 percent.

“Investors are taking out the prior winners one-by-one and taking them to the woodshed,” said Lew Piantedosi, vice president of growth equities at Eaton Vance Management in Boston, where he helps oversee almost $14 billion. “There’s been a bit of a flight to safety recently” as investors favor larger- cap stocks, he said.

Megacaps are obscuring weakening breadth in U.S. equities. To some, that’s an ominous sign amid market volatility that has seen the S&P 500 slide as much as 7.4 percent over the past two weeks, coming within five points of a 10-month low of 1,867.61 reached Aug. 25.

A version of the index that strips out market-weight biases, the S&P 500 Equal Weight Index, has already fallen past the level, closing on Monday and Tuesday below its Aug. 25 low of 2,931.78 before rebounding Wednesday.

Weighted for size, only two of the 10 major groups in the S&P 500, materials and health-care, are trading below the August threshold. That changes on an equal-weighted basis, as six industries, including consumer and energy stocks, have penetrated the level. Viewed like this, the benchmark gauge’s post-Aug. 25 rally dwindles to just 1.8 percent, rather than 2.8 percent when weighted for market cap.

Energy stocks have the biggest differential when gauging their performance, with the capitalization-weighted index up 4.4 percent versus 1.3 percent for the group on an equal-weighted basis.

Helping to keep this group afloat? Exxon, which has been “holding in there better than most of energy,” Piantedosi said. The stock has risen 8.2 percent since Aug. 25, after tumbling almost 26 percent in the preceding eight months.

Likewise, Amazon has buoyed consumer stocks, contributing to almost 20 percent of the sector’s post-August bounce. The group has gotten “really concentrated” in recent months after its run as one of the “big, persistent momentum leaders of the year,” said Jim Paulsen, the Minneapolis-based chief investment strategist at Wells Capital Management Inc., which oversees $351 billion.

Health-care stocks, meanwhile, offer a harbinger of what could happen more broadly, according to Paulsen. As investors piled into biotech, the high concentration in fewer companies has helped fuel a collapse, he said. The capitalization-weighted group now is trading 2.2 percent below its Aug. 25 trough.