Whatever the niche, there is probably a fund for it.

There are already exchange-traded funds that aim to profit from cannabis, the gig economy and adult obesity. One short-lived fund focused on trade and tariff fights. Another, the Cow exchange-traded note, bets on livestock.

These niche E.T.F.s are proliferating as fund managers try to stand out after a long bull market.

But whether investors really need these narrowly sliced funds is another question.

While broad- index mutual funds and E.T.F.s provide simple, low-cost ways of tracking the entire market, these so-called thematic E.T.F.s are relatively complex, costly and risky.

“This is a sector product, and the performance tends to be more volatile than a broad-market index product, and these funds tend to be on the more expensive side,” says Linda Zhang, chief executive of Purview Investments, an advisory firm that focuses on E.T.F.s. “For the individual investor, ask yourself: ‘Is this a marketing scheme or truly an investment?’”