  The 'expected utility hypothesis' and 'Savage's Sure-Thing Principle' are
violated in real life decisions, as shown by the 'Allais' and 'Ellsberg
paradoxes'. The popular explanation in terms of 'ambiguity aversion' is not
completely accepted. As a consequence, uncertainty is still problematical in
economics. To overcome these difficulties a distinction between 'risk' and
'ambiguity' has been introduced which depends on the existence of a
Kolmogorovian probabilistic structure modeling these uncertainties. On the
other hand, evidence of everyday life suggests that context plays a fundamental
role in human decisions under uncertainty. Moreover, it is well known from
physics that any probabilistic structure modeling contextual interactions
between entities structurally needs a non-Kolmogorovian framework admitting a
quantum-like representation. For this reason, we have recently introduced a
notion of 'contextual risk' to mathematically capture situations in which
ambiguity occurs. We prove in this paper that the contextual risk approach can
be applied to the Ellsberg paradox, and elaborate a sphere model within our
'hidden measurement formalism' which reveals that it is the overall conceptual
landscape that is responsible of the disagreement between actual human
decisions and the predictions of expected utility theory, which generates the
paradox. This result points to the presence of a quantum conceptual layer' in
human thought which is superposed to the usually assumed classical logical
layer', and conceptually supports the thesis of several authors suggesting the
presence of quantum structure in economics and decision theory.
