A record number of investors are investing in exchange-traded funds (ETFs). According to Morningstar, the industry reached a milestone with more than 3,000 ETFs trading simultaneously for the first time this month. This year, investors are adopting more active strategies, including single-stock ETFs, which provide traders with exposure to the daily performance of a single stock, such as Tesla or Apple. “We started off with basically taking very broad index funds — SPY [SPDR S&P 500 ETF Trust] was the first one — and then the industry over the years built all these interesting overlays,” Nick Colas, co-founder of DataTrek Research, told Bob Pisani on CNBC’s “ETF Edge” this week.

Included were sector and emerging funds, as well as funds specified to themes such as clean energy and legal marijuana, Colas said, as part of a shift from disruptive innovation to mainstream. “Investors now are really spoiled for choice among just being able to pick not only the big sector funds or the big overall funds but any kind of fund they think might be interesting,” he added. However, this move toward the specificity of thematic ETFs like cybersecurity ETFs has its risks, according to investment consultant Charles Ellis, the author of two upcoming books, “Inside Vanguard” and “Figuring It Out.” According to Ellis, those who invest in ETFs and then move into index funds will do well, while those who invest in highly specialized ETFs might make disastrous mistakes. “The more you get specific, the more the odds are high that you won’t be able to make a rational long-term decision, and you will get suckered into making, because we’re all human beings, a short-term emotional decision, and you won’t like the outcome in the long run,” Ellis said.