Why is this idea gaining traction now? Partly, it’s because of how the investment landscape has changed over the past couple of decades. There are only about half as many public companies in the United States today as there were in the late 1990s. And promising start-ups are tending to stay private longer, with elite investors capturing even more of the biggest gains.

There are reasons companies decide to stay private — including the more relaxed rules they face compared with public companies, which are required to make disclosures intended to allow investors to judge their value.

That lack of disclosure is just one of the reasons consumer advocates and others are wary of the idea. They also point to high fees associated with private investment funds, and the slim chances that smaller investors have at gaining early access to the next Google or Facebook.

And in the end, it might not even be worth the trouble. Over all, the performance of private funds doesn’t look much better than your standard mutual fund.

Private equity funds returned 13.3 percent in 2018 and 11.6 percent for the past 10 years ending in September 2018, after fees, according to PitchBook’s most recent private markets benchmark data. Investors who held mutual funds that specialized in small and midsize companies earned returns of 14.3 percent in 2018 and 9.4 percent over the same 10-year period.

But the spread between the best and worst performing private equity funds was much wider than for standard mutual funds: The top 25 percent performing private equity funds earned at least 16.2 percent over that 10-year period, while the bottom quartile returned at least 5.2 percent (the bottom 10 percent had negative returns). In the mutual fund world, the worst and best funds ranged from 9.8 percent (for the bottom 25 percent) to 12.2 percent (for the top 25 percent).

The S.E.C. acknowledged that it didn’t have a full picture of how investors fared in private investments, which are also called exempt offerings.