No evidence supports quantum decision theory

(Andrew W., PoliSci@Madison, Our place in the universe: Alexander Wendt and

quantum mechanics International Theory (2022), 14 (1), 130–145 doi:10.1017/S1752971921000063)

Chapter 8 focuses on quantum decision theory, what it means to be rational, and quantum game theory. Here, unlike in the case of vitalism, there are recent literatures to draw from. However, once again, Wendt is forthright in acknowledging that the scholars in these literatures usually explicitly deny the precise move Wendt wants to make: namely, to link their results to a theory of quantum brain function or quantum consciousness. So, despite the existing literatures, Wendt is out on a limb of his own. Quantum decision theory purports to explain experimental anomalies in expected utility theory (EUT). EUT assumes that people have fixed and transitive preferences or utilities and assess probabilities in accordance with classical probability theory, such as Bayes’ Rule, for updating beliefs in response to new information. They make choices to maximize their utility given their beliefs. Experimentalists, perhaps most famously Daniel Kahneman and Amos Tversky, have discovered many ways in which people depart from EUT. What people prefer often depends on how and in what order you ask the question. If you lead people to believe that Linda favors equal pay for women, they will think it more likely that she is a feminist and a bank teller than that she is a bank teller when the latter subsumes the former category and so must be more likely. People are more likely to gamble again if they know they won or lost the last gamble than if they remain uncertain about its outcome. Quantum decision theory purports to predict these deviations from classical rationality. The key claim of quantum decision theory is that there are no real underlying attitudes, beliefs, or intentions, just answers to questions that are produced by random processes which are modeled by quantum probability theory. Once again, nothing exists unless you measure it, and how you measure it changes what exists. To my mind this fails the introspection test, which, incidentally, Wendt sets great store by in a book about consciousness. It implies, for one thing, that if a person is alone in the woods, they have no social attitudes, because no one is asking them questions or presenting them with social decisions to make. To pick an example from the 2016 US Presidential election, does a Clinton supporter who shuts her office door suddenly become a possible Trump voter? Does a Catholic on his daily commute alone in his car become a possible Muslim because no one is there to check? Many social attitudes, identities, and so on, are highly predictable and resistant to change. It is hard to believe that quantum theory is going to improve our understanding of them even in cases where they are more volatile. But, what about the experiments? Doesn’t quantum decision theory predict the anomalies for EUT, just as quantum mechanics did for classical physics a century ago? I strongly suspect that it doesn’t. I grant that I am not able to review the literature that Wendt cites in depth for lack of time. However, until quantum decision theory achieves widespread acceptance among the scholars most closely concerned, in economics and psychology, I think the lay person has ample reason for doubt. The experiments that Wendt alludes to were very carefully crafted to be crucial tests of EUT. They are tests where EUT makes a clear prediction that can be falsified. It is not obvious to me that they function in a similar way for quantum decision theory. Does quantum decision theory make a specific prediction in these experiments that can be falsified by observed behavior? My guess is that it does not, but rather the quantum theory adds additional degrees of freedom to the model that render it supportable by any observed behavior in the experiment. For instance, imagine an experiment in which classical theory predicts a one to one, 45 degree relationship between two variables and quantum theory says it can take any angle depending on some unknown parameter alpha. Let’s say the angle turns out to be 10 degrees. Classical theory is falsified, but quantum theory is not confirmed, since it made no real prediction about the angle. Until quantum decision theory passes crucial experiments designed by economists to falsify it, I will remain skeptical about its claims.