The average expense ratio of hybrid mutual funds, which invest in a mix of equities and bonds, fell 1 basis point to 77 basis points in 2015. That's a smaller decline than that experienced by stock and bond funds.

Hybrid fund assets have risen considerably in the last few years; some of that growth has come in "alternative strategy" funds, which now account for 8 percent of the assets of all hybrid funds. Alternative strategy funds offer fund investors diversification across a wider range of asset classes and lower correlation with the equity market, but such strategies can be more costly to manage. The average expense ratio for other types of hybrid funds fell 2 basis points in 2015.

Money market fund expense ratios for 2015, at an average of 13 basis points, were unchanged from 2014. The current low interest rate environment has limited the expense ratios of money market funds over the last few years, as these funds have waived portions of their fees to prevent their net yields falling below zero. In 2015, 98 percent of money market fund share classes waived at least some portion of their fees. Fund advisers and their distributors pay for these waivers, which totaled an estimated $5.5 billion in 2015.

The average expense ratios for actively managed equity and bond funds fell by 2 and 3 basis points, respectively, in 2015. Competitive pressures and investors' interest in lower-cost funds helped to push down the cost of actively managed funds. For both actively managed and index funds, this demand for lower costs can be seen in the concentration of assets in the very lowest-cost funds. In 2015, 57 percent of the assets of actively managed equity funds were held in the 10 percent of such funds with the lowest expense ratios. In 2015, 69 percent of index equity fund assets were held in the 10 percent of index equity funds with the lowest expense ratios.