It seemed a mundane statement. MSCI, the New York-based index provider that runs the most widely accepted benchmarks of the emer-ging markets, had decided against a proposal to add China’s domestic A-shares to its main indices.

The company explained that China had still not shown that A-shares — open to non-Chinese only through a steadily expanding quota system — were sufficiently available to overseas investors. It also announced a joint working group with the China Securities Regulatory Commission with a view to adding A-shares by 2017. This innocuous sounding announcement turned out to be one of the catalysts for the huge sell-off in the Chinese stock market that sparked alarm around the world. It also offers a dramatic demonstration of the rising influence of the index.

Indices and the companies that calculate them have grown so powerful that they do not just track markets, but move them. With some critics contending that they help to inflate investment bubbles, the role that indexers play is facing closer scrutiny from regulators.