Socio-Cognitive Model of Trust: Quantitative Aspects - Trust Theory: A Socio-Cognitive and Computational Model - page 101

decision to delegate (to 'trust', as action): costs, risks, utility and so on. For example, the most
competent and trustworthy doctor might be the most expensive or not immediately available
(because very busy). In this case, we could delegate a less competent and cheaper one.
Another important thing to be underlined is that the complete scenario of Figure 3.4, with
all its branches and precise pros and cons of each alternative, is an ideal situation just rarely
effectively evaluated (by humans) in real life (and often also in artificially simulated scenarios):
it is more a normative model of a trust decision. However, increasing the importance of the
goal to be achieved, the risks of potential damages, and the time for the decision, the choice
of enquiring the potential alternative branches becomes a necessity beyond the available and
achievable information: it is the paradigmatic scenario upon which trust reasoning must be
based.

3.5 Positive Trust is not Enough: a Variable Threshold
for Risk Acceptance/Avoidance
As we saw, the decision to trust is based on some positive trust , i.e. on some evaluation and
expectation about the capability and willingness of the trustee and the probability of success.
And on the necessity/opportunity/preference of this delegation act.
First of all, those beliefs can be well justified, warranted and based on reasons. This
represents the 'rational' (reasons based) part of the trust in Y . But they can also be unwarranted,
not based on evidence, even quite irrational, or intuitive (based on sensations or feelings),
faithful. We call this part of the trust in Y : 'faith'. 11
Notice that irrationality in trust decision can derive from these unjustified beliefs, i.e. on the
ratio of mere faith.
Second, positive trust is not enough to account for the decision to trust/delegate. We do
not distinguish in this topic the different role or impact of the rational and irrational part of
our trust or positive expectations about Y 's action: the entire positive trust (reason-based +

faithful) is necessary and contributes to the Degree of Trust : its sum should be greater than
discouraging factors. We do not go deeply in this distinction (a part of the problem of rational
Vs irrational trust) also because we are interested here in the additional fact that this (grounded
or ungrounded) positive expectation can not be enough to explain the decision/act of trusting.
In fact, another aspect is necessarily involved in this decision: the decision to trust/delegate
necessarily implies the acceptance of some risk . A trusting agent is a risk-acceptant agent,
either consciously or unconsciously. Trust is never certainty: always it retains some uncertainty
(ignorance) 12 and some probability of failure, and the agent must accept this and be willing to
run such a risk (see Chapter 2) with both positive and negative expectations, and the fact that
they can remain just 'implicit' or 'potential'.
Thus a fundamental component of our decision to trust Y , is our acceptance and felt
exposition to a risk. Risk is represented in previous quantification of DoT and in the criteria

11 To be more precise, non-rational blind trust is close to faith. Faith is more than trust without evidence, it is trust
without the need for and the search for evidence, and even against evidence.
12 We do not want to introduce here a more sophisticated model where 'ignorance' and 'uncertainty' are explicitly
represented and distinct from probability; like the Dempster & Shafer model ((Dempster, 1968), (Shafer, 1976)) and
the theory of 'plausibility'. We use (less formally) this more sophisticated model in other chapters to explain trust
'optimism' and other aspects.