An Engine, Not a Camera
Summary
MacKenzie, a sociologist of science, argues that financial economics did not merely describe markets, it changed them. His central concept is performativity: a theory is performative when its use alters the world so that the world comes to resemble the theory.
His principal case is the Black-Scholes-Merton option pricing model. MacKenzie shows that before its publication, observed option prices differed substantially from what the model implied, and that after traders adopted it, prices converged toward the model's predictions. The convergence happened because traders used the model to identify and trade away discrepancies, and because the Chicago Board Options Exchange distributed model-derived sheets to floor traders. The theory made itself true. He documents the process through interviews with participants and through price data.
He covers the wider history of financial economics including Markowitz, Modigliani and Miller, Sharpe, Fama and the efficient market work, and treats each as an intervention in markets rather than only a description of them. He also examines the reverse case: the 1987 crash, after which the volatility smile appeared and never disappeared, meaning the market permanently stopped conforming to the model. He calls this counterperformativity, where use of a theory makes it less true. Long-Term Capital Management gets similar treatment.
The book is a scholarly work in science studies and is written for that audience, though the finance is explained clearly. Its argument is genuinely important for investors: models used widely enough change what they are modelling.
Key takeaways 7
- A theory is performative when its use changes the world so that the world comes to resemble the theory, and financial economics has repeatedly done this.
- Observed option prices differed substantially from Black-Scholes before its adoption and converged toward it afterwards, because traders used the model to arbitrage discrepancies.
- The Chicago Board Options Exchange distributed model-derived price sheets to floor traders, which is a concrete mechanism by which a theory entered prices.
- Financial models used widely enough change what they are modelling, which means their empirical validation is partly self-generated.
- The volatility smile that appeared after 1987 and never disappeared is evidence of counterperformativity, where use of a model made it less accurate.
- Portfolio insurance failed because widespread adoption of the same hedging rule created the correlated selling the rule assumed would not occur.
- Financial theory should be understood as an intervention in markets rather than as a description of them, which changes how its empirical record should be read.
Who should read it
Investors and quantitative practitioners who need to understand how widely adopted models change the markets they describe.
Who recommended it 1 · 1 direct
“his book An Engine Not a Camera (which I would highly recommend if you are interested in these topics)”
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