This is the first study to examine both how well plan administrators select funds for 401(k) plans and how participants react to plan administrator decisions. We find that, on average, administrators select funds that outperform randomly selected funds of the same type although they do not outperform index funds of the same type. When administrators change offerings, they choose funds that did well in the past, but, after the change, added funds do no better than dropped funds. Plan participants in aggregate change their allocation decisions in a way that accentuates the changes in allocation caused by returns. The change in allocation due to the investment of new money and interfund transfers is about the same size, and in the same direction, as the change due to returns. Participant allocations in aggregate do no better than naïve allocation rules, such as equal investment in each offering.

Many investors confine their mutual fund holdings to a single fund family either for simplicity or through restrictions placed by their retirement savings plan. We find evidence that mutual fund returns are more closely correlated within than between fund families. As a result, restricting investment to one fund family leads to a greater total portfolio risk than diversifying across fund families. We examine the sources of this increased correlation and find that it is due primarily to common stock holdings, but is also more generally related to families having similar exposures to economic sectors or industries. Fund families also show a propensity to focus on high or low risk strategies, which leads to a greater dispersion of risk across restricted investors. An investor considering adding an additional fund, either in the same family or outside the family, would need to believe the inside fund offered an extra 50 to 70 basis points to have the same Sharpe ratio.