The engineering of stock market indices: winners and losers

Stock market indices are among the important figures that are followed in the trading rooms: on the movements of the Dow Jones or the S&P 500 depends the allocation of billions of dollars. At the same time, for the companies that produce and publish them, but also for those whose shares are included in them, they represent a significant source of visibility – and therefore of income. Based on an ethnographic survey at the Brussels stock exchange, this article documents how the different financial actors – aware of the performativity of stock market indices – try to impact their shape. The divergent opinions and interests of these actors are the source of methodological tensions at the heart of these numbers. Through the methods of calculation and selection chosen, the engineers of the indices make perilous trade-offs with far-reaching consequences. By exposing the cognitive and political debates that these indicators crystallise, this article also highlights the unequal representation of the various stakeholders within these statistics. This type of inequality, veiled by the numerical formatting, often goes under the radar of social scientists.