  This study utilises an experiment famous in quantum physics, the
Stern-Gerlach experiment, to inform the structure of an experimental protocol
from which a quantum cognitive decision model can be developed. The
'quantumness' of this model is tested by computing a discrete
quasi-probabilistic Wigner function. Based on theory from quantum physics, our
hypothesis is that the Stern-Gerlach protocol will admit negative values in the
Wigner function, thus signalling that the cognitive decision model is quantum.
A crowdsourced experiment of two images was used to collect decisions around
three questions related to image trustworthiness. The resultant data was used
to instantiate the quantum model and compute the Wigner function. Negative
values in the Wigner functions of both images were encountered, thus
substantiating our hypothesis. Findings also revealed that the quantum
cognitive model was a more accurate predictor of decisions when compared to
predictions computed using Bayes' rule.
