NEW YORK, Aug. 25 -- Most of the money in American retirement accounts is invested according to a list that a committee decides. Not an algorithm, not a market-capitalisation cutoff applied automatically, but a group of people at S&P Dow Jones Indices who meet, apply written criteria, and exercise judgment about which 500 companies belong.

The clearest demonstration is sitting in plain view. SpaceX is one of the largest companies listed in the United States. It joined the Nasdaq 100 within a month of its debut. It is not in the S&P 500 and will not be for years, because the S&P 500 requires a company to have earned money, and SpaceX has not.

The index itself fell 0.28 per cent to 7,652.79 on Monday afternoon. Its own equal-weighted vers...