In addition, they can expose the sponsor to fiduciary liability if those fees are spread among participants who don’t use the windows in their accounts.

Given the lack of sophistication of the average American investor, a brokerage window could present not an opportunity, but a hazard to most people participating in a retirement plan.

The need for better understanding of the investments accessible through a window is beyond the skills of most workers, while the people who do use brokerage windows are often in particular professions, such as doctors and lawyers.

In December, Schwab data provided in comments to the DOL on the subject of brokerage windows indicated that, while more than 60 percent of the plans it services offer windows, just 4 percent of participants use them.

However, they still manage to account for 11.8 percent of plan assets.

Even more extraordinary is information from a new Vanguard paper that examined the self-directed brokerage feature in the plans it provides.

It found that, in 2014, 28 percent of Vanguard plan participants had access to brokerage windows—but only 1 percent actually used them. In fact, 10 percent of the firms offering brokerage windows had zero participants using them.

In addition, it said, 22 percent of the firms with access to brokerage windows were law firms, and “on average 7 percent of law firm assets were invested in brokerage.”

Brokerage window users are different from other plan participants in several ways, Vanguard said. “Brokerage participants are 6 years older, have more than twice the tenure, and are disproportionately male [80 percent male], compared with the Vanguard universe [59 percent male],” the paper said. “Brokerage participants contacted Vanguard much more frequently than all participants did—with a median contact rate of 66 times in 2014, compared with only 2 contacts in 2014 for all participants.”

However, here’s the kicker: “The most striking difference between brokerage participants and all participants is their account balances. The median brokerage account balance was $262,000—more than 8 times larger than the median Vanguard account balance of $30,000.”

Brokerage window participants, it said, invest an average of 45 percent of their account balances in the brokerage option. While larger plans are more likely to provide a brokerage window to their participants, “smaller firms have a higher proportion of plan assets invested in the brokerage option....” The brokerage allocation ranges from 10 percent or less (18 percent of brokerage participants) to more than 90 percent (15 percent of participants).