Financial-market index-makers are growing in power

IT WAS in 1896 that Charles Dow, co-founder of Dow Jones & Company, created the index that still bears his name. Today, indices such as the Dow Jones Industrial Average and the S&P 500 (for shares listed in New York), or the FTSE 100 (for London), are among the best-known brands in financial markets. The role they play has expanded massively in recent years. Index-makers have become finance’s new kingmakers: arbiters of how investors should allocate their money.

Stockmarket indices were devised as a measure of the overall market, against which those trading in shares could compare their performance. At first they were concocted by the press or by exchanges themselves. For bonds, indices were compiled by the banks that traded them. Except for a few of the very earliest indices, such as the Dow, which is weighted by share price, nearly all are weighted by market capitalisation or, in the case of bond indices, by the volume of debt outstanding.

Three large firms—…