Some of the overt resistance to indexing by active, stock-picking managers turned to covert concession. If, as was increasingly the case, a manager's performance was judged relative to an index such as the S&P 500, then there was some safety in selecting a portfolio that closely resembled the makeup of the index. Of course, doing so meant little or no chance of a dramatic overperformance. However, it also greatly lessened the chances of a career-killing relative underperformance: if one's portfolio did badly, those of other managers would most likely be doing badly too, so the fault would be seen to lie with the market, not the manager.

Donald MacKenzie, An Engine, Not a Camera, loc. 381