  Ambiguity and ambiguity aversion have been widely studied in decision theory
and economics both at a theoretical and an experimental level. After Ellsberg's
seminal studies challenging subjective expected utility theory (SEUT), several
(mainly normative) approaches have been put forward to reproduce ambiguity
aversion and Ellsberg-type preferences. However, Machina and other authors have
pointed out some fundamental difficulties of these generalizations of SEUT to
cope with some variants of Ellsberg's thought experiments, which has recently
been experimentally confirmed. Starting from our quantum modeling approach to
human cognition, we develop here a general probabilistic framework to model
human decisions under uncertainty. We show that our quantum theoretical model
faithfully represents different sets of data collected on both the Ellsberg and
the Machina paradox situations, and is flexible enough to describe different
subjective attitudes with respect to ambiguity. Our approach opens the way
toward a quantum-based generalization of expected utility theory (QEUT), where
subjective probabilities depend on the state of the conceptual entity at play
and its interaction with the decision-maker, while preferences between acts are
determined by the maximization of this 'state-dependent expected utility'.
