Factor investing continues to be a popular theme in the exchange-traded fund world, and the current single-factor flavor of the month is ETFs comprised of stocks with low volatility.

Generally, factor investing picks stocks with similar characteristics that have shown to produce higher returns over time, such as investing in high-quality stocks or investing in small-cap stocks over large caps. Low-volatility ETFs group stocks that traditionally exhibit fewer price swings than the broader market over time and usually have higher yields.

One of the perceived benefits of low-volatility investing is that during times of big price swings and steep falls in valuation these investments have limited losses, although the vast majority of these have never been tested in real bear markets such as 2008.

The low-vol craze has been fueled by seemingly relentless market volatility. This year alone has been buffeted by the first-quarter swoon in the broader markets, the post-Brexit tumult after the U.K. voted to leave the European Union, concerns over China’s economy, or any number of other headline risks.

“Investors who are looking to sleep better at night were really drawn to these strategies,” says David Mazza, head of ETF and mutual fund research at State Street Global Advisors.

Many low-vol ETFs hold names that investors equate with stability, such as General Mills, Johnson & Johnson and Procter & Gamble, says Todd Rosenbluth, ETF and mutual funds research director at S&P Global Market Intelligence.

Michael Krause, president of ETF Research Center, says he counted 32 ETFs using the keywords “low volatility,” not including leveraged ETFs, while ETFdb.com counts 44 such funds. Regardless of the final number, the top three—iShares Edge MSCI Min Vol USA ETF (USMV), iShares Edge MSCI Min Vol EAFE ETF (EFAV) and PowerShares S&P 500 Low Volatility Portfolio (SPLV)—hold 70 percent of all assets under management for this category, which Krause estimates at around $45 billion.

Research from State Street Global Advisors suggests this strategy gathered about $16.05 billion year-to-date globally, up sharply from 2015’s inflows of $10.89 billion. Of this year’s total, about $6.8 billion went to domestic low-volatility ETFs.

Performance is strong. Several of the biggest low-volatility ETFs based on assets under management sport year-to-date returns between 8 and 12 percent, versus the S&P 500 year-to-date return of about 6.47 percent and the MSCI World Index return of 3.66 percent.

With numbers like that, it’s easy to see why investors want a piece of the action. But is the trade getting crowded?

It’s a good question considering these funds focus on defensive sectors such as healthcare, consumer staples and real estate investment trusts that have seen big gains this year as investors seek dividend-yielding stocks in a low-interest rate environment.

The three funds above choose traditionally stable stocks, but approach indexing differently, Rosenbluth says. The iShares approach picks mostly large-cap stocks, but seeks some diversity across sectors. USMV is a domestic-focused fund and the low-vol category’s largest with $14.6 billion in AUM. It’s up 10 percent year-to-date. EFAV has a global focus, holds $8.06 billion in AUM and is up 3.2 percent

SPLV, which has $7.66 billion in AUM and is up 8.66 percent, contains 100 of the least volatile stocks within the S&P 500 index. USMV and EFAV rebalance every six months and SPLV rebalances quarterly.

“It’s logical that there is concern some of these ETFs have gotten too much attention,” Rosenbluth says. But neither Rosenbluth nor Krause believe fund flows are affecting performance.

Not only are the underlying stocks much larger than the ETFs, Krause notes, but that overall these stocks aren’t necessarily heavily owned in the ETF universe.

Mazza says he’s less worried about this being a crowded trade, considering another very popular single-factor strategy—dividend-paying ETFs—have about $115 billion in AUM. Although low-volatility ETFs are seeing big inflows, they’ve come from a low asset base so the asset flows do not indicate overcrowding.

Valuation, though, is another thing, the three researchers say. Mazza says State Street Global Advisors research shows current low-volatility factor valuations are getting to be a bit rich versus the past. However, he says the valuations aren’t as stretched as they have been during some timeframes like the global financial crisis or the European debt crisis. During those times, the low-volatility factor became expensive because investors bought up low-volatility stocks to try to reduce price swings and drawdown risk of their portfolios, which pushed up their valuations.

Rosenbluth says financial advisors and investors need to remember the purpose of these funds. “The goal of the [low-vol] ETF isn’t to outperform; it’s to be less volatile,” he says. “Advisors should be using these as a way to get equity exposure with a lower risk profile and not just picking it because it’s outperforming. Its outperformance will be fleeting.”