When savers are offered the option of a brokerage window, they are used sparingly. PSCA data shows that only 1.3 percent of total plan assets are accounted for by investments through brokerage windows.

When a sponsor does offer a brokerage window, it must be made available to all of a plan's participants. The vastness of investment options offered through windows, combined with the risk that less financially savvy participants could misappropriate the option, has left many in the plan advisory and legal community leery of the feature, lest it expose employers to expanded fiduciary liability.

The question of whether only financially savvier participants are using the accounts could be found in the wealth that is aggregating in SDBAs.

The most recent data from Charles Schwab, a leading provider of SDBAs, shows that average balance for advised brokerage accounts is $436,709. For non-advised accounts, the average balance is $226,283. About half of the assets invested through Schwab's brokerage window offering are advised accounts.

Comparatively, the average 401(k) account balance in 2017 was $102,280, according to PSCA.

Boomers have the highest average balance, at $365,561; Gen Xers average brokerage window balance is $194,534; and millennials' average account holds $61,916.

Of the 137,000 SDBAs that Schwab pulls quarterly data from, 41.6 percent are held Boomers, 40 percent are held Gen Xers, and 10.7 percent are held by Millennials.

Mutual funds account for 38 percent of brokerage window investments, followed by individual equities at 28.7 percent, ETFs at 16.9 percent, cash equivalents at 13.9 percent, and fixed income at 2.3 percent.

Participants hold an average of 9.7 investments through Schwab's brokerage window platform. Millennials held the highest percentage of cash, at 16 percent of their holdings.

Here is a look at the top 10 individual equity holdings, as a percentage of assets, that investors, by generation, are tapping through Schwab's window:

10. Percentage of assets: 1.12 – 1.33%

• Millennials hold 1.12 percent of SDBA assets in Alphabet (C share)
• Gen Xers hold 1.33 percent of assets in Microsoft
• Boomers hold 1.18 percent in Bank of America

9. Percentage of assets: 1.13 – 1.42%

• Millennials hold 1.13 percent of SDBA assets in Alphabet (A share)
• Gen Xers hold 1.42 percent of assets in Tesla
• Boomers hold 1.21 percent in Alibaba

8. Percentage of assets: 1.29 – 1.49%

• Millennials hold 1.49 percent of SDBA assets in Nvidia
• Gen Xers hold 1.45 percent of assets in Alibaba
• Boomers hold 1.29 percent in Alphabet (Google)

7. Percentage of assets: 1.38 – 1.5%

• Millennials hold 1.5 percent of SDBA assets in Alibaba
• Gen Xers hold 1.47 percent of assets in Nvidia
• Boomers hold 1.38 percent in Nvidia

6. Percentage of assets: 1.55 – 1.81%

• Millennials hold 1.81 percent of SDBA assets in Berkshire Hathaway
• Gen Xers hold 1.55 percent of assets in Bank of America
• Boomers hold 1.79 percent in Netflix

5. Percentage of assets: 1.84 - 2.81%

• Millennials hold 2.81 percent of SDBA assets in Netflix
• Gen Xers hold 2.37 percent of assets in Berkshire Hathaway
• Boomers hold 1.84 percent in Microsoft

4. Percentage of assets: 2 – 3.04%

• Millennials hold 3.04 percent of SDBA assets in Tesla
• Gen Xers hold 2.52 percent of assets in Netflix
• Boomers hold 2 percent in Facebook

3. Percentage of assets: 2.58 – 3.16%

• Millennials hold 3.16 percent of SDBA assets in Facebook
• Gen Xers hold 2.91 percent of assets in Facebook
• Boomers hold 2.58 percent in Berkshire Hathaway

2. Percentage of assets: 5 – 6.93%

• Millennials hold 5.73 percent of SDBA assets in Apple
• Gen Xers hold 6.93 percent of assets in Amazon
• Boomers hold 5 percent in Amazon

1. Percentage of assets: 8.65 – 9.61%

• Millennials hold 7.6 percent of SDBA assets in Amazon
• Gen Xers hold 9.61 percent of assets in Apple
• Boomers hold 8.65 percent in Apple