This book has, however, also emphasized that counterperformativity is possible: that practical action based on economic models can undermine the empirical validity of those models. One instance noted above is the way in which the development of index funds an expression of the efficient-market view that systematic success in stock picking is unlikely seems to have created an anomaly from the viewpoint of efficient-market theory: the increases in price that typically followed inclusion of a stock in a major index. However, by far the most significant possible instance of counterperformativity is the role of portfolio insurance (which was based, albeit loosely, on a Black-Scholes world, in other words a world in which the assumptions of the Black-Scholes-Merton model were valid) in exacerbating the 1987 crash and thus undermining the Black-Scholes world.

Donald MacKenzie, An Engine, Not a Camera, loc. 1166