(Bloomberg) — Private equity firms are good at solving complex problems. What's stumping them? Normal people.

Five years ago, Carlyle Group LP's David Rubenstein predicted a future where ordinary savers would be able to invest in private equity, an industry limited to wealthy individuals and institutions. He later suggested that by now, Americans would be able to put some of their 401(k) retirement accounts into the asset class.

Today most mom-and-pop investors still don't have that option. But a shift in how people are saving in their 401(k)s may give private equity a new way in — keeping firms like Carlyle, Blackstone Group LP and KKR & Co. eyeing the $4.8 trillion that U.S. workers have saved in their 401(k)s.