But as the traditional 401(k) holdings have stagnated, the "alts" have performed better. One Georgetown University study cited by the bill's sponsors estimated a modest and conservative diversification into alts could boost 401(k) performance by 17% a year and cut losses in a recession.

Toomey's bill, which amends the Employee Retirement Income Security Act of 1974,  "clarifies" for 401(k) managers actions they can safely take to increase returns for future retirees without risking litigation.

Related: When clients need guidance on how to invest

Three central points to the bill include:

• The clarification that "plan fiduciaries may select investment options that include a range of asset classes, including private equity." The sponsors note that ERISA does not limit the asset classes for 401(k) plans. Rather, managers have simply avoided them. The bill  "makes clear that Congress intends to let investment professionals determine the appropriate range of asset classes."
• Full protection for ERISA's fiduciary standard, no safe harbor for the fiduciary. Although ERISA's fiduciary standards have been validated by a U.S. Supreme Court ruling, the bill stipulates that "fiduciaries must still select investments through a prudent process."
• Promotion of prudent diversification of retirement savings plans. The bill recognizes that plan managers may not want to diversify. But to encourage them to do it, it provides "tools" to facilitate diversification.

Surveys of plan managers, particularly those overseeing 401(k) retirement plans, have shown them to be essentially conservative and reluctant to alter their holdings mix. While the bill tacitly includes digital investment vehicles (bitcoin) in the alt mix, sponsors don't anticipate a rush to add such holdings to a retirement fund portfolio.