T he economic role of actively managed equity mutual funds is to delegate the stock selection decisions of individual investors to professional fund managers. The general belief is that these fund managers can generate abnormal returns relative to passive investment strategies. There is an ongoing debate among academics regarding whether actively managed funds outperform passive mutual funds or index funds. Many studies have found that active fund managers do not outperform the market-for example, Wermers (2000) showed that, on average, actively managed funds do not outperform the market after fees and expenses.The finding that the average actively managed fund does not generate abnormal returns has led researchers to investigate what limits the ability of fund managers to generate performance. For example, Ackermann and Ravenscraft (1999) focused on regulatory restrictions faced by funds; Ellis (2014) argued that the lack of private information is a hindrance. A recent strand of research has examined fund or manager characteristics that affect mutual fund performance in order to identify which mutual funds perform better. 1 Our study builds on the literature that has identified portfolio concentration as a key dimension that affects performance (e.g., Brands, Brown, and Gallagher 2005; Kacperczyk, Sialm, and Zheng 2005; Ivković, Sialm, and Weisbenner 2008; Cohen, Polk, and Silli 2010; Huij and Derwall 2011) and extends it in several directions. First, many of these studies measured the concentration of portfolios on the basis of concentration across industry sectors, but we found an additional, marginal impact on performance by within-sector concentration. This finding strengthens the conclusion that concentration has more to do with mutual fund ability than with picking up abnormal returns associated with industry momentum strategies, which also lead to industryconcentrated portfolios (see Wu 2015). Second and perhaps more important, we identified the organizational design behind the loss of abnormal returns associated with less concentrated portfolios. In particular, we found that mutual funds run by a single manager tend to have a much higher portfolio concentration, both across and within industries, than funds run by multiple managers. We further found that when funds' management designs are changed from single manager to multiple managers (or from multiple to single), portfolio concentration decreases (increases) and performance deteriorates (improves).To construct measures of mutual fund portfolio concentration, we followed Kacperczyk et al. (2005), who first showed that mutual funds with portfolios concentrated in a few industry sectors tend to outperform. They argued that this cross-sector concentration is an indication of fund managers' self-assurance and ability. Similarly, we constructed an industry concentration index (ICI) on the basis of 10 industry sectors. 2 In addition, we developed a new within-sector concentration index (WCI) to capture the degree of mutual fund portfolio concentration withi...
