Sales charges are a very subtle form of behavioral coaching: High fees discourage active trading, and most people are terrible active traders. The presence of a sizable front- or back-end load encourages people to stay invested so the returns keep compounding. There have been lots of studies that show how fees eat into returns over time. This is simple math. All else equal, if the expense ratio on your mutual fund is 1 percent, you will have less money at some point in the future than if you had put your money in a fund with an expense ratio of 0.1 percent.