We examine diseconomies of scale for two different investment approaches: quantitative and fundamental. Using separate account (SA) data where the investment approach is self-identified, we find that fundamental SAs exhibit greater diseconomies of scale than quantitative SAs. Looking at liquidity costs, we find that quantitative SAs hold more diversified portfolios of higher liquidity stocks than fundamental SAs, thereby reducing their expected liquidity costs. We also find that consistent with lower information processing/hierarchy costs, the speed of information diffusion is higher for quant SAs. Accounting for these differences helps to explain the differences in diseconomies of scale.

After analyzing portfolio differences between separate account‐mutual fund twins, we find that dissimilar “fraternal twins” show significantly lower joint performance than “identical twins.” This finding is consistent with fraternal twins competing for the limited attention of a manager while identical twins mutually profit. Furthermore, the effect is stronger for separate accounts, which is probably due to investors having the opportunity to influence managers’ investment decisions according to their preferences. These results are independent of differences in known investment constraints. However, the findings may be driven by separate account investors’ preferences for higher liquidity and lower idiosyncratic risk.

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