Mutual funds that track the S&P 500 are popular because they have significantly lower costs than the average, actively managed equity fund. However, a measurable number of investors select index funds with excessive fees and uncompetitive returns. We call this observation the Index Fund Rationality Paradox because it conflicts with the belief that index fund investors are making a rational, low-cost choice in their ‘type of fund’ decision. In our analysis of this paradox, we find that both retail and institutional index investors tended to make better choices in recent years, but the cost of poor choices among both groups continues to be significant. In fact, we are able to identify an arguably naïve group of retail investors that seem to be unduly influenced by brokers and financial advisors. These investors are largely responsible for the remaining paradox.

Given recent regulatory inquiries into the derivative-trading practices of mutual funds, we examine their detailed option holdings to assess how mutual funds employ options, what funds use options, and how that affects performance and risk. Mutual funds’ use of options appears consistent with income generation and hedging motives, is systematically related to experience, education, and gender characteristics of portfolio managers, and does not lead to performance benefits, on average. Instead, certain uses of options lead to underperformance. We document no permanent or temporary aggressive risk taking by options users, finding instead that some funds use options to effectively lower risk.

Firms’ competitive advantages are unsustainable when competitors hire their employees away to study and recreate those advantages. We document inter-firm knowledge spillovers through labor mobility in the mutual fund industry, which result in performance improvement at the recipient family. This effect intensifies when frictions hampering knowledge absorption at the recipient family are weaker and switching managers had better access to the organization processes at the originating family. Performance deterioration at the originating family, which intensifies when more money chases the newly-transferred knowledge, suggests erosion of its competitive advantage and wealth transfers across investors in the respective families.

This study examines how the efficiency of trading desks operated by mutual fund families affects portfolio performance and investment behavior of affiliated funds. We estimate the trading efficiency of a fund family's trading desk as the difference between the gross return of the family's index fund, which incorporates trading costs, and the return of the underlying index, which does not incorporate trading costs, around index adjustment dates. By operating more efficient trading desks that help reduce trading costs, fund families improve the performance of their funds significantly and also enable their funds to trade more and hold less liquid portfolios.

We examine how labor mobility restrictions in the form of non-compete clauses in employment contracts affect employee behavior. Using the mutual fund industry as testing laboratory, we show that fund managers respond to higher job termination costs due to increased enforceability of non-compete clauses by increasing their contribution to their employer's revenue. They do so by improving their fund performance, while also increasing window dressing to attract new customers and increase fee revenue. Furthermore, the change in incentives disciplines managers’ risk taking, as shown by noticeable reductions in their portfolio risk, portfolio deviations from peers, and engagement in fund tournaments.